GLX_SGIX_swap_barrier
http://oss.sgi.com/projects/ogl-sample/registry/SGIX/swap_barrier.txt
	void glXBindSwapBarrierSGIX (Display *dpy, GLXDrawable drawable, int barrier)
	Bool glXQueryMaxSwapBarriersSGIX (Display *dpy, int screen, int *max)
