prevent race condition around framebuffers

src/frontend/qt_sdl/main.h

@@ -26,6 +26,7 @@
 #include <QImage>
 #include <QActionGroup>
 #include <QTimer>
+#include <QMutex>
 
 #include <QOffscreenSurface>
 #include <QOpenGLWidget>
@@ -59,6 +60,9 @@ public:
 
     bool emuIsRunning();
 
+    int FrontBuffer = 0;
+    QMutex FrontBufferLock;
+
 signals:
     void windowUpdate();
     void windowTitleChange(QString title);