iidx: don't hook legacy camera unless requested (#856)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #855 

## Description of change
Legacy camera hooks are specifically for IIDX 25/26 and they only
perform redirection of device discovery (emulates USB vendor/device ID
and USB port). It has very limited use since most cameras are
practically unusable in IIDX 25/26. Don't enable it, unless explicitly
requested by the user.

New truth table for `-iidxcabcams` :

| Mode | `-iidx` off (cab setup) | `-iidx` on |
|---|---|---|
| `auto` | Native cameras enabled (same as `on`) | Cameras disabled
(same as `off`) |
| `off` | Cameras disabled | Cameras disabled |
| `on` | Native cameras enabled | Native cameras enabled |
| `legacy` | Native cameras enabled | IIDX 25/26 discovery emulation
enabled |

## Testing

src/spice2x/games/iidx/iidx.h

@@ -11,6 +11,13 @@
 
 namespace games::iidx {
 
+    enum class cab_camera_access_mode {
+        automatic,
+        off,
+        on,
+        legacy,
+    };
+
     enum class iidx_aio_emulation_state {
         unknown,
         bi2a_com2,
@@ -20,7 +27,7 @@ namespace games::iidx {
     // settings
 
     extern bool FLIP_CAMS;
-    extern std::optional<bool> DISABLE_CAMS;
+    extern cab_camera_access_mode CAB_CAMERA_ACCESS;
     extern bool TDJ_CAMERA;
     extern bool TDJ_CAMERA_PREFER_16_9;
     extern std::optional<std::string> TDJ_CAMERA_OVERRIDE;