Fix analogue filter allocating/freeing surfaces every frame

This removes memory churn caused by using analogue mode.

The surfaces are only allocated if analogue mode is turned on, and kept
after they are initialized. Otherwise, if analogue mode is never turned
on (which will be the case for the vast majority of the time the game is
played), then no extra memory is used.

desktop_version/src/GraphicsUtil.h

@@ -12,6 +12,6 @@ void DrawPixel(SDL_Surface* surface, int x, int y, SDL_Color color);
 SDL_Color ReadPixel(const SDL_Surface* surface, int x, int y);
 
 void UpdateFilter(void);
-void ApplyFilter(void);
+void ApplyFilter(SDL_Surface* src, SDL_Surface* dest);
 
 #endif /* GRAPHICSUTIL_H */