Fix dirty framebuffer issues in fullscreen TIA mode.

This requires some testing, since it potentially causes slowdowns on older systems.
I cannot find any difference on my test system (and documentation I've read seems to imply
that you should clear the screen before each update, and that it is very fast on most hardware).

src/emucore/FrameBuffer.cxx

@@ -424,6 +424,7 @@ void FrameBuffer::updateInEmulationMode(float framesPerSecond)
   // We don't worry about selective rendering here; the rendering
   // always happens at the full framerate
 
+  clear();  // TODO - test this: it may cause slowdowns on older systems
   myTIASurface->render();
 
   // Show frame statistics