overlay: mouse-as-touch needs to check if subscreen window is active (#842)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #840 

## Description of change
Currently, for subscreen games, mouse events are still delivered even
when the subscreen overlay window is not visible.

Change this so that the subscreen overlay must be active and under the
mouse cursor for the mouse-to-touch transformation to occur. Applies to
both native and wintouchemu.

## Testing
Needs to test everything again..

iidx:

- [ ]  full screen with overlay
- [ ]  single window with overlay
- [ ]  two window

Test: mouse, poke, api, real touch screen

sdvx:

- [ ] full screen with overlay
- [ ] windowed

popn

- [x] full screen with overlay
- [x] window with overlay
- [x] dedicated window

gitadora

- [x] single window with overlay
- [x] dedicated sub window

nostalgia

- [x] fullscreen
- [x] windowed

test: poke

wintouchemu
- [ ] Do all of the above again with wintouchemu

src/spice2x/overlay/overlay.h

@@ -109,6 +109,7 @@ namespace overlay {
         void show_main_menu();
         void set_active(bool active);
         bool get_active();
+        bool accepts_subscreen_mouse_input();
         bool has_focus();
         bool hotkeys_triggered();
 
@@ -143,20 +144,20 @@ namespace overlay {
             return this->hWnd;
         }
 
-        bool can_transform_touch_input() {
-            return (this->subscreen_mouse_handler != nullptr);
+        bool has_subscreen_touch_transform() {
+            return (this->subscreen_touch_transform != nullptr);
         }
 
         bool transform_touch_point(LONG *x, LONG *y) {
-            if (this->get_active() && this->subscreen_mouse_handler) {
-                return this->subscreen_mouse_handler(x, y);
+            if (this->get_active() && this->subscreen_touch_transform) {
+                return this->subscreen_touch_transform(x, y);
             } else {
                 return true;
             }
         }
 
-        void set_subscreen_mouse_handler(const std::function<bool(LONG *, LONG *)> &f) {
-            this->subscreen_mouse_handler = f;
+        void set_subscreen_touch_transform(const std::function<bool(LONG *, LONG *)> &f) {
+            this->subscreen_touch_transform = f;
         }
 
         // renderer
@@ -203,7 +204,7 @@ namespace overlay {
 
         std::vector<std::unique_ptr<Window>> windows;
 
-        std::function<bool(LONG *, LONG *)> subscreen_mouse_handler = nullptr;
+        std::function<bool(LONG *, LONG *)> subscreen_touch_transform = nullptr;
 
         bool active = false;
         bool toggle_down = false;
@@ -216,6 +217,7 @@ namespace overlay {
         // so render() never runs without a matching NewFrame.
         bool has_pending_frame = false;
 
+        bool is_subscreen_overlay_visible();
         void init();
         void add_font(const char* font, ImFontConfig* config, const ImWchar* glyphs);
     };