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_UpdateRect(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.