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");
     }