[feat] add native Metal video rendering for Slint on macOS

src/gui/application/gui_application.h

@@ -41,6 +41,10 @@ private:
   void SyncStreamVideoFrame();
   void ScheduleNextVideoFrame();
   void ConfigureStreamVideoRenderer();
+#if defined(__APPLE__)
+  void SubmitCachedFrameToMetal(
+      const std::shared_ptr<RemoteSession>& session);
+#endif
   void SyncStreamKeyboardFocus();
   void SetStreamKeyboardFocus(bool focused);
   void SyncServerWindow();