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't support double-buffering, this is equivalent
to calling SDL.update_rect(screen, 0, 0, 0, 0)
The SDL.DOUBLEBUF flag must have been passed to
SDL_SetVideoMode, when setting the video mode for this
function to perform hardware flipping.