sdk: introduce Spice SDK (#676)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds a flat-C, headers-only library for writing DLL plugins with spice.

This is meant to be an alternative to writing applications over spice
API which doesn't suit some scenarios like writing custom
high-performance I/O modules.

All you have to do is:

1. Create a 32-bit or 64-bit DLL with `spice_sdk_entry_point` as a
DLLExport
1. Include `spicesdk.h`, and optionally `spicesdk_io.h` (see
`extras\sdk\include`)
1. `spice_sdk_entry_point` will be called when hooks get initialized.
Call `init` with valid parameters.
1. When `init` returns, `SPICE_SDK_V0` will be filled out with a series
of function pointers; your DLL can now start calling into it.
1. Compile your DLL and add it as a DLL hook (`-k`) in spice when
launching the game.

See `v0_cpp.cpp` for an example.

V0 implements the following functions:

```
    log;

    get_game_info;
    get_avs_info;

    get_button;
    set_button;

    get_analog;
    set_analog;

    get_light;
    set_light;

    set_touch;
    clear_touch;

    insert_card;
    set_keypad;
```

V0 is focused on providing common utility functions needed in a lot of
DLL hooks (get AVS info, log messages to file), and implementing I/O.

In the future, we could have more interesting things like:

* scanning and modifying memory 
* installing DLL hooks
* having direct hooks into I/O emulation modules
* getting full tape LEDs
* having a callback into DX9 `Present` call
* drawing an ImGui window

More detailed documentation is coming soon in form of a wiki page.

## Testing
See included sample DLL.

src/spice2x/sdk/include/spicesdk.h

@@ -0,0 +1,273 @@
+#pragma once
+#ifndef SPICE_SDK_H
+#define SPICE_SDK_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#ifdef __cplusplus
+#define SPICE_SDK_ENTRY_POINT extern "C" __declspec(dllexport) int __cdecl
+#else
+#define SPICE_SDK_ENTRY_POINT __declspec(dllexport) int __cdecl
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef enum SPICE_SDK_STATUS_CODE {
+    SPICE_SDK_STATUS_SUCCESS = 0,
+
+    // 000: generic
+    SPICE_SDK_STATUS_GENERIC_ERROR = 1,
+    SPICE_SDK_STATUS_NOT_INITIALIZED = 2,
+    SPICE_SDK_STATUS_NOT_SUPPORTED = 3,
+    SPICE_SDK_STATUS_TOO_SMALL = 4,
+    SPICE_SDK_STATUS_TOO_LATE = 5,
+
+    // 1000: invalid args
+    SPICE_SDK_STATUS_INVALID_ARGUMENT_1 = 1001,
+    SPICE_SDK_STATUS_INVALID_ARGUMENT_2 = 1002,
+    SPICE_SDK_STATUS_INVALID_ARGUMENT_3 = 1003,
+    SPICE_SDK_STATUS_INVALID_ARGUMENT_4 = 1004,
+    SPICE_SDK_STATUS_INVALID_ARGUMENT_5 = 1005,
+
+} SPICE_SDK_STATUS_CODE;
+
+typedef enum SPICE_SDK_LOG_LEVEL {
+    SPICE_SDK_LOG_LEVEL_MISC = 0,
+    SPICE_SDK_LOG_LEVEL_INFO = 1,
+    SPICE_SDK_LOG_LEVEL_WARNING = 2,
+    SPICE_SDK_LOG_LEVEL_FATAL = 3,
+} SPICE_SDK_LOG_LEVEL;
+
+typedef struct SPICE_SDK_TOUCH_POINT {
+    uint32_t id;
+    int x;
+    int y;
+} SPICE_SDK_TOUCH_POINT;
+
+typedef struct SPICE_SDK_GAME_INFO {
+    char name[64]; // null-terminated
+} SPICE_SDK_GAME_INFO;
+
+typedef struct SPICE_SDK_AVS_INFO {
+    char model[4]; // "MDX", null-terminated
+    char dest;     // J
+    char spec;     // A
+    char rev;      // A
+    char ext[11];  // "2025061002", null-terminated
+} SPICE_SDK_AVS_INFO;
+
+// get_game_info (v0.1 and up)
+//
+// get info about the currently running game
+//
+//   info: receives game info
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_get_game_info_func)(
+    SPICE_SDK_GAME_INFO *info
+);
+
+// get_avs_info (v0.1 and up)
+//
+// get AVS info (model, dest, spec, rev, ext)
+//
+//   info: receives AVS info
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_get_avs_info_func)(
+    SPICE_SDK_AVS_INFO *info
+);
+
+// log (v0.1 and up)
+// logs a message to the log
+// writing a FATAL message will terminate spice, only use in catastrophic failure
+// 
+//   level: see log level enum
+//   module: short string that identifies the facility / module / submodule
+//   message: the message to log
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_log_func)(
+    SPICE_SDK_LOG_LEVEL level,
+    const char *module,
+    const char *message
+);
+
+// get_button (v0.1 and up)
+// gets the button state
+//
+//   button_id: ID of the button; see spicesdk_io.h for named values
+//   pressed: (optional) is the button pressed?
+//   velocity: (optional) MIDI velocity of the button
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_get_button_func)(
+    uint32_t button_id,
+    bool *pressed,
+    float *velocity
+);
+
+// set_button (v0.1 and up)
+// sets or clears the button override
+//
+// make sure to hold the button long enough for the game's I/O engine to pick up
+// usually, one or two frames
+//
+//   button_id: ID of the button; see spicesdk_io.h for named values
+//   pressed: true to set the button override (permanently set the button to be ON until cleared),
+//            false to clear the override (allow user's controller to provide input again)
+//   velocity: MIDI velocity of the button; only valid when pressed is true
+//             can be between 0.0 and 1.0, inclusive
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_set_button_func)(
+    uint32_t button_id,
+    bool pressed,
+    float velocity
+);
+
+// get_analog (v0.1 and up)
+// gets the analog state
+//
+//   button_id: ID of the button; see spicesdk_io.h for named values
+//   value: receives the state of the analog
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_get_analog_func)(
+    uint32_t analog_id,
+    float *value
+);
+
+// set_analog (v0.1 and up)
+// sets or clears the analog override
+//
+//   analog_id: ID of the analog; see spicesdk_io.h for named values
+//   override_active: true to set override (gain exclusive control)
+//                    false to clear it (allow user's controller provide input again)
+//   value: value of the analog; only valid when override_active is true
+//          can be between 0.0 and 1.0, inclusive
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_set_analog_func)(
+    uint32_t analog_id,
+    bool override_active,
+    float value
+);
+
+// get_light (v0.1 and up)
+// gets the last observed value of a light
+//
+//   light_id: ID of the light; see spicesdk_io.h for named values
+//   value: output parameter for the light value; 0.0 to 1.0
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_get_light_func)(
+    uint32_t light_id,
+    float *value
+);
+
+// set_light (v0.1 and up)
+// sets or clears the light override
+//
+//   light_id: ID of the light; see spicesdk_io.h for named values
+//   light_value: output parameter for the light value; 0.0 to 1.0
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_set_light_func)(
+    uint32_t light_id,
+    bool override_active,
+    float light_value
+);
+
+// set_touch (v0.1 and up)
+// adds or updates touch points
+//
+//   points: array of touch points to add or update
+//   count: number of touch points in the array
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_set_touch_func)(
+    const SPICE_SDK_TOUCH_POINT *points,
+    uint32_t count
+);
+
+// clear_touch (v0.1 and up)
+// clears touch points (i.e., no longer being touched)
+//
+//   ids: array of touch point IDs to clear
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_clear_touch_func)(
+    const uint32_t *ids,
+    uint32_t count
+);
+
+// insert_card (v0.1 and up)
+// simulates inserting an e-amuse card with the given ID
+//
+//   unit: 0 for player 1, 1 for player 2
+//   card_id: null-terminated string of the card ID
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_insert_card_func)(
+    uint8_t unit,
+    const char *card_id
+);
+
+// set_keypad (v0.1 and up)
+// sets keypad state
+//
+// make sure to hold the button long enough for the game to pick up
+// 70ms is usually sufficient, except for DDR which needs 150ms
+//
+//   unit: 0 for player 1, 1 for player 2
+//   key: '0' to '9' for numbers, 'A' for 00, 'D' for decimal point, 0 or '\0' to release all keys
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_set_keypad_func)(
+    uint8_t unit,
+    char key
+);
+
+typedef struct SPICE_SDK_V0 {
+    uint32_t size;
+
+    spice_sdk_log_func *log;
+
+    spice_sdk_get_game_info_func *get_game_info;
+    spice_sdk_get_avs_info_func *get_avs_info;
+
+    spice_sdk_get_button_func *get_button;
+    spice_sdk_set_button_func *set_button;
+
+    spice_sdk_get_analog_func *get_analog;
+    spice_sdk_set_analog_func *set_analog;
+
+    spice_sdk_get_light_func *get_light;
+    spice_sdk_set_light_func *set_light;
+
+    spice_sdk_set_touch_func *set_touch;
+    spice_sdk_clear_touch_func *clear_touch;
+
+    spice_sdk_insert_card_func *insert_card;
+    spice_sdk_set_keypad_func *set_keypad;
+
+} SPICE_SDK_V0;
+
+typedef void (__cdecl spice_sdk_destroy_callback_func)(
+    void
+);
+
+// init (v0.1 and up)
+//
+//   version: supply 0
+//   destroy_callback: supply a function pointer that will be called when spice
+//                     is shutting down
+//   sdk_functions: supply a pointer to SPICE_SDK_V0; ensure size field is initialized
+//                  to sizeof(SPICE_SDK_V0) before calling this function
+
+typedef SPICE_SDK_STATUS_CODE (__cdecl spice_sdk_init_func)(
+    uint32_t version,
+    spice_sdk_destroy_callback_func *destroy_callback,
+    void *sdk_functions
+);
+
+typedef int (__cdecl spice_sdk_entry_point_func)(
+    spice_sdk_init_func *init
+);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif // SPICE_SDK_H