gitadora: allow a landscape monitor to display subscreen (#899)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
Arena Model's SMALL touch subscreen is a portrait (800x1280) panel -
however, it's common for people to set up a touch screen in landscape
orientation since IIDX/SDVX/popn all use landscape.

Add a new option, `-gdsublandscape`, which forces the subscreen to
launch at 1080p landscape. The image is rendered with black bars on the
sides. Touches will be unaligned initially; user needs to go into test
mode and recalibrate. In reality this option is just a short hand for
`-forceressub 1920,1080`.

Wire up `-forceressub` which previously didn't work for gitadora, so
that a resolution other than 1080p can be used in either orientation. In
other games `-forceressub` just forces the rendering resolution and
IIDX/SDVX/popn's game engine automatically stretches to fill... however
for gitadora it works differently since we want to keep the aspect ratio
of the original screen; it instead hijacks the back buffer and makes the
game draw to an intermediate buffer.

## Testing

src/spice2x/launcher/launcher.cpp

@@ -693,6 +693,9 @@ int main_implementation(int argc, char *argv[]) {
     if (options[launcher::Options::GitaDoraArenaRealtekAccess].value_bool()) {
         games::gitadora::ALLOW_REALTEK_AUDIO = true;
     }
+    if (options[launcher::Options::GitaDoraSubscreenLandscape].value_bool()) {
+        games::gitadora::ARENA_SUBSCREEN_LANDSCAPE = true;
+    }
     if (options[launcher::Options::LoadNostalgiaModule].value_bool()) {
         attach_nostalgia = true;
     }