Don't extern gameScreen, use *graphics.screenbuffer instead

Externing gameScreen is just unnecessary. Still a good idea to have it
off the stack, though.

desktop_version/src/Screen.h

@@ -33,6 +33,4 @@ public:
 	SDL_Rect filterSubrect;
 };
 
-extern Screen gameScreen;
-
 #endif /* SCREEN_H */