ScreenPanelNative: Don't try to render the framebuffer if the emulator is not actually active.

This fixes an issue where the window draws with a gray background in macOS, and is see-through on Wayland-based desktops.

src/frontend/qt_sdl/main.h

@@ -59,6 +59,7 @@ public:
     void emuFrameStep();
 
     bool emuIsRunning();
+    bool emuIsActive();
 
     int FrontBuffer = 0;
     QMutex FrontBufferLock;