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/launcher/options.cpp

@@ -1056,9 +1056,12 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
     },
     {
         // GitaDoraArenaSingleWindow
-        .title = "GitaDora Arena Single Window (EXPERIMENTAL)",
+        .title = "GitaDora Arena Disable Subscreens (EXPERIMENTAL)",
         .name = "gdaonewindow",
-        .desc = "For Arena Model, when combined with windowed mode, only show the main game window; enables subscreen overlay.",
+        .desc = "For Arena Model:\n\n"
+            "Windowed mode: instead of 4 windows, create 1 window.\n\n"
+            "Fullscreen mode: instead of requiring 4 monitors, use only the primary monitor. WARNING: requires 4K monitor.\n\n"
+            "To access the subscreen, use the subscreen overlay.",
         .type = OptionType::Bool,
         .game_name = "GitaDora",
         .category = "Game Options",