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/window.h

@@ -24,6 +24,7 @@ namespace overlay {
         void toggle_active();
         void set_active(bool active);
         bool get_active();
+        bool is_mouse_hovered();
 
         // raise this window to the top of the z-order on its next build. only
         // call in response to an explicit user action (keyboard toggle, UI
@@ -37,6 +38,7 @@ namespace overlay {
         // set when the window transitions to visible so build() raises it to the
         // top of the z-order on the next frame
         bool request_focus = false;
+        bool mouse_hovered = false;
         std::vector<Window*> children;
 
         // settings