Qt: Ensure fullscreen UI is stopped on exiting

src/duckstation-qt/mainwindow.cpp

@@ -714,6 +714,10 @@ void MainWindow::quit()
       QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
   }
 
+  // Big picture might still be active.
+  if (m_display_created)
+    g_emu_thread->stopFullscreenUI();
+
   // Ensure subwindows are removed before quitting. That way the log window cancelling
   // the close event won't cancel the quit process.
   destroySubWindows();