Qt/RenderWidget: Restore size when exiting fullscreen

Source/Core/DolphinQt2/MainWindow.h

@@ -191,4 +191,5 @@ private:
   RegisterWidget* m_register_widget;
   WatchWidget* m_watch_widget;
   CheatsManager* m_cheats_manager;
+  QSize m_render_widget_size{640, 480};
 };