sdl_flip — Swaps screen buffers
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).
SDL_DOUBLEBUF flag must have been passed to
when setting the video mode for this function to perform hardware flipping.