Make sure SDL_CaptureMouse() is only called on the main thread

Windows handles mouse capture on a per-thread basis, and capture must be done on the thread used to create a window.

Fixes https://github.com/libsdl-org/SDL/issues/5577

src/events/SDL_mouse.c

@@ -1069,6 +1069,17 @@ SDL_CaptureMouse(SDL_bool enabled)
         return SDL_Unsupported();
     }
 
+#ifdef __WIN32__
+    /* Windows mouse capture is tied to the current thread, and must be called
+     * from the thread that created the window being captured. Since we update
+     * the mouse capture state from the event processing, any application state
+     * changes must be processed on that thread as well.
+     */
+    if (!SDL_OnVideoThread()) {
+        return SDL_SetError("SDL_CaptureMouse() must be called on the main thread");
+    }
+#endif /* __WIN32__ */
+
     if (enabled && SDL_GetKeyboardFocus() == NULL) {
         return SDL_SetError("No window has focus");
     }

src/video/SDL_sysvideo.h

@@ -333,6 +333,7 @@ struct SDL_VideoDevice
 
     /* * * */
     /* Data common to all drivers */
+    SDL_threadID thread;
     SDL_bool checked_texture_framebuffer;
     SDL_bool is_dummy;
     SDL_bool suspend_screensaver;
@@ -461,6 +462,8 @@ extern VideoBootStrap NGAGE_bootstrap;
 extern VideoBootStrap OS2DIVE_bootstrap;
 extern VideoBootStrap OS2VMAN_bootstrap;
 
+/* Use SDL_OnVideoThread() sparingly, to avoid regressions in use cases that currently happen to work */
+extern SDL_bool SDL_OnVideoThread(void);
 extern SDL_VideoDevice *SDL_GetVideoDevice(void);
 extern int SDL_AddBasicVideoDisplay(const SDL_DisplayMode * desktop_mode);
 extern int SDL_AddVideoDisplay(const SDL_VideoDisplay * display, SDL_bool send_event);

src/video/SDL_video.c

@@ -470,6 +470,7 @@ SDL_VideoInit(const char *driver_name)
     _this = video;
     _this->name = bootstrap[i]->name;
     _this->next_object_id = 1;
+    _this->thread = SDL_ThreadID();
 
 
     /* Set some very sane GL defaults */
@@ -549,6 +550,12 @@ SDL_GetVideoDevice(void)
     return _this;
 }
 
+SDL_bool
+SDL_OnVideoThread()
+{
+    return (_this && SDL_ThreadID() == _this->thread) ? SDL_TRUE : SDL_FALSE;
+}
+
 int
 SDL_AddBasicVideoDisplay(const SDL_DisplayMode * desktop_mode)
 {