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/overlay/windows/exitprompt.cpp

@@ -71,7 +71,7 @@ namespace overlay::windows {
             sub = "Show Valkyrie Subscreen";
         } else if (avs::game::is_model("REC")) {
             sub = "Show DRS Dance Floor";
-        } else if (games::gitadora::is_arena_model() && games::gitadora::ARENA_SINGLE_WINDOW) {
+        } else if (games::gitadora::is_arena_model()) {
             sub = "Show GITADORA Subscreen";
         } else if (games::popn::is_pikapika_model()) {
             sub = "Show Pop'n Subscreen";