overlay: refactor overlay layering (#739)

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

## Description of change

Create three distinct layers for the overlay:

1. Bottommost persistent layer - non-interactable layer that is always
on. This was only for notifications, but now the FPS widget lives here
as well.
2. Overlay windows layer - most interactable windows go here.
3. Topmost main menu - this is reserved for the main menu (escape key)
and this is a modal dialog that occludes the layers below.

Why? 

- `toggle overlay` behavior with FPS widget *also* toggling on/off was a
bit confusing (now they're two separate keys)
- FPS widget is popular, but it caused the entire overlay to be active,
which affects how input is handled
- the main menu being a standalone window was a little awkward (now it's
a modal)

## Testing

src/spice2x/overlay/overlay.h

@@ -29,6 +29,14 @@ namespace overlay {
         SOFTWARE,
     };
 
+    // corner of the screen the FPS / clock / timer window is anchored to
+    enum class FpsLocation {
+        TopLeft,
+        TopRight,
+        BottomLeft,
+        BottomRight,
+    };
+
     // settings
     extern bool ENABLED;
     extern bool AUTO_SHOW_FPS;
@@ -37,7 +45,7 @@ namespace overlay {
     extern bool AUTO_SHOW_KEYPAD_P1;
     extern bool AUTO_SHOW_KEYPAD_P2;
     extern bool USE_WM_CHAR_FOR_IMGUI_CHAR_INPUT;
-    extern bool FPS_SHOULD_FLIP;
+    extern FpsLocation FPS_LOCATION;
     extern bool SHOW_DEBUG_LOG_WINDOW;
     extern std::optional<uint32_t> UI_SCALE_PERCENT;
 
@@ -57,10 +65,8 @@ namespace overlay {
 
         D3DDEVICE_CREATION_PARAMETERS creation_parameters {};
         D3DADAPTER_IDENTIFIER9 adapter_identifier {};
-        bool hotkeys_enable = true;
 
         // windows
-        Window *window_fps = nullptr;
         Window *window_iopanel = nullptr;
         Window *window_config = nullptr;
         Window *window_keypad1 = nullptr;
@@ -72,6 +78,15 @@ namespace overlay {
         Window *window_sub = nullptr;
         Window *window_log = nullptr;
 
+        // not part of `windows`: drawn/updated on the persistent layer (like
+        // notifications), independent of the overlay's active state and input gates.
+        std::unique_ptr<Window> window_fps;
+
+        // not part of `windows`: the main menu / launcher. owned and drawn
+        // separately from the overlay window layer; it drives the overlay's
+        // active state while shown so its buttons still receive input.
+        std::unique_ptr<Window> window_main_menu;
+
         explicit SpiceOverlay(HWND hWnd, IDirect3D9 *d3d, IDirect3DDevice9 *device);
 #ifdef SPICE_D3D11
         explicit SpiceOverlay(HWND hWnd, ID3D11Device *d3d11_device,
@@ -87,7 +102,7 @@ namespace overlay {
         // after render() when the frame is being presented. In-game overlay draws in render().
         void d3d9_render_draw(bool force_submit = false);
         void update();
-        void toggle_active(bool overlay_key = false);
+        void toggle_active();
         void show_main_menu();
         void set_active(bool active);
         bool get_active();
@@ -182,13 +197,12 @@ namespace overlay {
 
         std::vector<std::unique_ptr<Window>> windows;
 
-        Window *window_main_menu = nullptr;
-
         std::function<bool(LONG *, LONG *)> subscreen_mouse_handler = nullptr;
 
         bool active = false;
         bool toggle_down = false;
         bool main_menu_down = false;
+        bool fps_down = false;
         bool hotkey_toggle = false;
         bool hotkey_toggle_last = false;