Add ClearSurface()

This is a function that does what it says - it clears the given surface.
This just means doing a FillRect(), but it's better to use this function
because it conveys intent better.

desktop_version/src/GraphicsUtil.h

@@ -35,6 +35,8 @@ void FillRect( SDL_Surface* surface, SDL_Rect& rect, const int r, int g, int b )
 
 void FillRect( SDL_Surface* surface, SDL_Rect rect, int rgba );
 
+void ClearSurface(SDL_Surface* surface);
+
 void ScrollSurface(SDL_Surface* _src, int pX, int py);
 
 SDL_Surface * FlipSurfaceVerticle(SDL_Surface* _src);