void SDL_UpdateRect(SDL_Surface *screen, Sint32 x, Sint32 y, Sint32 w, Sint32 h);
Makes sure the given area is updated on the given screen. The rectangle must be confined within the screen boundaries (no clipping is done).
If 'x', 'y', 'w' and 'h' are all 0, SDL_UpdateRect will update the entire screen.
This function should not be called while 'screen' is locked.