This uses CVDisplayLink to drive the GdkFrameClock. A GdkWindow
can register a frame callback to thaw their frame clock as necessary
based on the next notification from CVDisplayLink.

CVDisplayLink notifies us on a high-priority thread. We use the same
NSEventas gdkeventloop-quartz.c to wakeup the main loop. This is done
so that we don’t pathologically wake up the select thread to then
continue notifying the main loop.

We use an embedded GList node in the GdkWindowImplQuartz so that we
can avoid allocating any lists or arrays for pending frame callbacks.
Compare this to the same design in GdkWindow for children.