Interpolate bringing up and down quit/pause/teleporter screen

Now it's really, really smooth. Except for like the last frame when it
goes down, which I sometimes didn't notice (but maybe it didn't happen
every time due to being lucky on the delta timesteps or something,
whatevs.)

desktop_version/src/Graphics.h

@@ -255,6 +255,7 @@ public:
 	bool backgrounddrawn, foregrounddrawn;
 
 	int menuoffset;
+	int oldmenuoffset;
 	bool resumegamemode;
 
 	SDL_Rect warprect;