Problem 1: fullscreen switching was leaving artifacts. Fixed by
drawing all black when switching fullscreen (and switching fullscreen
is now handled by its own function).
Problem 2: the mission briefing screen would distort if you changed
fullscreen during it. Fixed by redrawing the screen (it was previously
only drawn once).
I think this should be good, but I can't test it right now because
I don't have a Windows compiler handy at the moment. Will make sure
to do so before making a release.