compositor: Base zoom update or refresh rate

Bug: 15391

Previously, the zoom update was based on XRandr refresh rate ÷ 2, or
30 by default if not available.

But that XRandr code was removed along with the removal of the fake
vblank sync code in favor of XPresent or GLX.

Base the zoom update on half the refresh rate of the primary monitor (if
available from GDK), so we would get the same results as with 4.12.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>