
extern int SDL_Flip(SDL_Surface *screen);
On hardware that supports double-buffering, this function sets up a flip
and returns. The hardware will wait for vertical retrace, and then swap
video buffers before the next video surface blit or lock will return.
On hardware that doesn not support double-buffering, this is equivalent
to calling SDL_UpdateRect
(screen, 0, 0, 0, 0)
;