Move horizontal/vertical warp backgrounds to separate buffers

Instead of using the same tower buffer that gets used for towers, use a
separate buffer instead so there's no risk of stepping on the tower
buffer's toes at the wrong point in time.

This commit combined with the previous one fixes #369.

desktop_version/src/Graphics.h

@@ -228,6 +228,8 @@ public:
 	SDL_Surface* menubuffer;
 	SDL_Surface* foregroundBuffer;
 	SDL_Surface* tempBuffer;
+	SDL_Surface* warpbuffer;
+	SDL_Surface* warpbuffer_lerp;
 
 	TowerBG towerbg;
 	TowerBG titlebg;