gitadora: (arena model) single window mode + subscreen overlay (#486)

## Link to GitHub Issue, if one exists
#477 

## Description of change
This is a work in progress.

What works:
- [x] subscreen overlay shows the touch screen image (in windowed mode)
- [x] touch works in the overlay
- [x] only the main window is shown, other windows are not launched

What doesn't:
- [ ] game occasionally crashes when entering test menu?
- [ ] full screen mode is not tested at all due to monitor requirements
- [ ] need more polish (various options to control the behavior)

src/spice2x/overlay/windows/gfdm_sub.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "overlay/window.h"
+#include "overlay/windows/generic_sub.h"
+
+namespace overlay::windows {
+
+    class GitaDoraSubScreen : public GenericSubScreen {
+    public:
+        GitaDoraSubScreen(SpiceOverlay *overlay);
+
+    protected:
+        void touch_transform(const ImVec2 xy_in, LONG *x_out, LONG *y_out) override;
+
+    private:
+        static void keep_10_by_16(ImGuiSizeCallbackData* data) {
+            data->DesiredSize.y = (data->DesiredSize.x * 16.f / 10.f) + ImGui::GetFrameHeight();
+        }
+    };
+}