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.cpp

@@ -69,6 +69,7 @@ void overlay::Window::build() {
 
     // check if active
     if (!this->active) {
+        this->mouse_hovered = false;
         return;
     }
 
@@ -106,10 +107,18 @@ void overlay::Window::build() {
         // top spot. suppress that so only an explicit focus request (see below)
         // decides what comes to the front.
         const std::string window_id = this->title + "###" + to_string(this);
-        if (ImGui::Begin(
+        const bool build_contents = ImGui::Begin(
                 window_id.c_str(),
                 &this->active,
-                this->flags | ImGuiWindowFlags_NoFocusOnAppearing)) {
+                this->flags | ImGuiWindowFlags_NoFocusOnAppearing);
+
+        // keep the window hovered while its invisible input button is held during a drag
+        this->mouse_hovered =
+            ImGui::IsWindowHovered(
+                ImGuiHoveredFlags_RootAndChildWindows |
+                ImGuiHoveredFlags_AllowWhenBlockedByActiveItem);
+
+        if (build_contents) {
 
             // window attributes - init_pos / init_size are only honored once
             // (ImGuiCond_Once), so compute them a single time instead of every frame
@@ -154,6 +163,9 @@ void overlay::Window::build() {
 
     } else {
 
+        // raw content does not have an ImGui window to hover
+        this->mouse_hovered = false;
+
         // add raw content
         this->build_content();
     }
@@ -201,3 +213,7 @@ bool overlay::Window::get_active() {
     // now it depends on us
     return this->active;
 }
+
+bool overlay::Window::is_mouse_hovered() {
+    return this->mouse_hovered;
+}