clutter: Smooth out master clock to smooth visuals

clutter: Smooth out master clock to smooth visuals

Clutter's master clock was jittery because it included errors in cur_tick
such as dispatch delays due to other sources. Dispatch could also occur up
to 1ms early since GSource can only be timed to the millisecond. All of this
could impact the visual smoothness of animations as they are displayed on
the steady interval of the monitor, but spacially moving in less regular
steps derived from the dispatch times.

The simple fix is to ignore any jitter in dispatch timing. Try a little
bit harder to use a precise interval that will better match the display
hardware, and smoother visuals will follow.

Bug: https://gitlab.gnome.org/GNOME/mutter/issues/25
Forwarded: https://gitlab.gnome.org/GNOME/mutter/merge_requests/70

Gbp-Pq: Name clutter-Smooth-out-master-clock-to-smooth-visuals.patch