gitadora: (arena model) single window mode + subscreen overlay (#486)

## Link to GitHub Issue, if one exists
#477 

## Description of change
This is a work in progress.

What works:
- [x] subscreen overlay shows the touch screen image (in windowed mode)
- [x] touch works in the overlay
- [x] only the main window is shown, other windows are not launched

What doesn't:
- [ ] game occasionally crashes when entering test menu?
- [ ] full screen mode is not tested at all due to monitor requirements
- [ ] need more polish (various options to control the behavior)

src/spice2x/CMakeLists.txt

@@ -554,6 +554,7 @@ set(SOURCE_FILES ${SOURCE_FILES}
         overlay/windows/camera_control.cpp
         overlay/windows/card_manager.cpp
         overlay/windows/drs_dancefloor.cpp
+        overlay/windows/gfdm_sub.cpp
         overlay/windows/screen_resize.cpp
         overlay/windows/sdvx_sub.cpp
         overlay/windows/config.cpp