Drop SGI_video_sync based method for vsync

With the new compositing timing, we want to start compositing some time
later after a vsync event. This doesn't go along with the video sync
based method to synchronize buffer swaps with vblank.

Since practically all drivers nowadays provide support for the swap
control extensions (GLX_EXT_swap_control, GLX_SGI_swap_control, or
GLX_MESA_swap_control), it's safe to rely on them for the purpose of
synchronizing buffer swaps to vblank.