Previously clutter timelines advanced according to
But that meant the spatial stepping of animations was visibly sensitive to
any irregularities in the main loop. It also represented a time older 
than the intended presentation time of each frame.
Now we instead use
master_clock_get_next_presentation_time. This ensures
we get the 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 on the lock screen etc.
 "older" = (refresh_interval - sync_delay) = ~14ms for 60Hz
Closes: #25 (closed)