iidx: hide tdj sub screen in fullscreen single monitor mode (#829)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Prevent the game from launching a blank window when launching fullscreen
with sub disabled.

## Testing

src/spice2x/launcher/launcher.cpp

@@ -379,6 +379,7 @@ int main_implementation(int argc, char *argv[]) {
     }
     if (options[launcher::Options::spice2x_IIDXNoSub].value_bool()) {
         GRAPHICS_FORCE_SINGLE_ADAPTER = true;
+        GRAPHICS_PREVENT_SECONDARY_WINDOWS = true;
     }
     if (options[launcher::Options::spice2x_SDVXNoSub].value_bool()) {
         GRAPHICS_FORCE_SINGLE_ADAPTER = true;