Sync timelines to hardware vsync
Previously clutter timelines advanced according to
But that meant they were subject to dispatch erraticness and represented
a time older  than the final presentation time.
Now we instead use
provides the near perfect smoothness of hardware vsync as well as being
closer to the actual presentation time.
This means, for example, backends like Xorg that move the hardware cursor independently of repaints will have their animations more closely matching the hardware cursor position. So the cursor appears to stick more closely when dragging windows or the lock screen etc.
 "older" = (refresh_interval - sync_delay) = ~14ms for 60Hz