Previously clutter timelines advanced according to
But that meant the spatial stepping of animations was visibly sensitive to
any irregularities in the main loop. Now we instead use the intended update time. This ensures we get the smoothness of hardware vsync.
Closes: #25 (closed)
Relates to !724 (merged) which landed in 3.36 only. The difference is that !1190 (closed) doesn't rely on backend presentation timestamps so should be easier to integrate with the planned changes in 3.38.