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,
The SDL_DOUBLEBUF flag must have been passed to
SDL_!SetVideoMode?, when setting the video mode for
this function to perform hardware flipping.
This function returns 0 if successful, or -1
if there was an error.