Instead we just cache the monitor number and get
out of it the nsscreen when it is needed. This is
a requirement since it nsscreen it is not supposed
to be cached.
Fixes: https://gitlab.gnome.org/GNOME/gtk/issues/1312
The addition of GdkMonitor broke the quartz backend. This patch restores
that support by adding a new class GdkQuartzMonitor, and by modifying
the existing classes GdkQuartzDisplay and GdkQuartzScreen where
necessary.
It should be noted that this patch is essentially a refactor as no new
functionality that will impact the user has been added or removed.
https://bugzilla.gnome.org/show_bug.cgi?id=779184