drs: rgb camera hook (#524)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Add plug&play hooks for RGB camera in DRS.

## Testing
Seems to work with my crappy Logitech C270

Camera options (brightness etc) seem to work in test menu as well).

Tested by another user with realsense camera + logitech camera, both
cams were detected correctly.

src/spice2x/games/drs/drs.h

@@ -31,6 +31,7 @@ namespace games::drs {
     extern std::vector<TouchEvent> TOUCH_EVENTS;
     extern bool DISABLE_TOUCH;
     extern bool TRANSPOSE_TOUCH;
+    extern bool RGB_CAMERA_HOOK;
 
     void fire_touches(drs_touch_t *events, size_t event_count);
     void start_touch();