Move analogue mode filter update logic to fixed-timestep loop

Otherwise if the analogue filter is scrolling, it'll scroll by REALLY
fast.

desktop_version/src/GraphicsUtil.h

@@ -46,6 +46,7 @@ void ScrollSurface(SDL_Surface* _src, int pX, int py);
 SDL_Surface * FlipSurfaceHorizontal(SDL_Surface* _src);
 SDL_Surface * FlipSurfaceVerticle(SDL_Surface* _src);
 SDL_Surface * ScaleSurfaceSlow( SDL_Surface *_surface, int Width, int Height );
+void UpdateFilter();
 SDL_Surface* ApplyFilter( SDL_Surface* _src );
 
 #endif /* GRAPHICSUTIL_H */