Move title usages of towerbg to titlebg

With the previous commit in place, we can now simply move some usages of
the previous towerbg to use a separate object instead. That way, we
don't have to mess with a monolithic state, or some better way to phrase
what I just said, and we instead have two separate objects that can
coexist side-by-side.

desktop_version/src/Graphics.h

@@ -230,6 +230,7 @@ public:
 	SDL_Surface* tempBuffer;
 
 	TowerBG towerbg;
+	TowerBG titlebg;
 
 	SDL_Rect bfont_rect;
 	SDL_Rect tiles_rect;