Qt: Ensure fullscreen UI is stopped on exiting

Backport from: 533022bb7d

pcsx2-qt/MainWindow.cpp

@@ -586,6 +586,10 @@ 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();