macos: support mix-rate monitors

macos: support mix-rate monitors

Previously, a single CVDisplayLink was used to drive updates for all
surfaces across all monitors. It used a 'best guess' rate which would
allow for updates across monitors of mixed rates. This is undesirable for
situations where you might have a 144hz monitor as it does not allow for
reaching up to that frame rate.

Instead, we want to use a per-monitor CVDisplayLink which will fire at the
rate of the monitor down to the level of updates we require. This commit
does just that.

When a surface crosses onto a new monitor, that monitor is used to drive
the GdkFrameClock.

Fixes #4732