gitadora: (arena model) booting fullscreen with one monitor (#696)

## Link to GitHub Issue or related Pull Request, if one exists
#477 

## Description of change
By default the game needs 4 monitors.

With the new option to disable subscreens, you can now boot fullscreen
with a single 4K monitor, with subscreen overlay enabled. Side monitors
are not shown in the overlay; only the touch screen is emulated.

## Future work

- [ ] custom resolution (-forceres) doesn't work properly (hooking
ResetEx does allow the game to boot but will render incorrectly;
probably needs hex edits)
- [x] enable this automatically if the user has fewer than 4 monitors
- [ ] ability to boot with a 4k main screen + small subscreen (with no
side screens which do nothing gameplay wise), but this whole display
emulation is a nightmare to work with & test thoroughly so it'll have to
be in the future.


## Testing
Checklist:

- [x] fs on 1 monitor with option set
- [x] fs on 2 monitor with option set
- [x] windowed - default (4 windows, no overlay)
- [x] windowed - with option set (1 window, overlay)
- [x] fs with 4 monitors?
- [x] check popn hc for regression

src/spice2x/games/gitadora/gitadora.h

@@ -11,7 +11,6 @@ namespace games::gitadora {
     // settings
     extern bool TWOCHANNEL;
     extern std::optional<unsigned int> CAB_TYPE;
-    extern bool ARENA_SINGLE_WINDOW;
     extern bool P1_LEFTY;
     extern bool P2_LEFTY;
     extern std::optional<std::string> SUBSCREEN_OVERLAY_SIZE;