We need to coordinate with MetaCompositor during pre-paint so that we
have control over whether MetaLater callbacks happen first, or the
MetaCompositor pre-paint logic.
In order to do so, make MetaLater listen to a new signal "pre-paint" on
MetaCompositor, that is called MetaCompositors own pre-paint handling.
This fixes an issue where the top window actor was calculated after the
MetaCompositor pre-paint handling, meaning the top actor being painted
was out-of-date.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/798
Since the order of destruction during MetaDisplay tear down is a bit
unordered, there are pieces that try to destruct its compositing
dependent pieces (i.e. queued MetaLater callbacks) after MetaCompositor
has been cleaned up, meaning we need to put some slightly awkward NULL
checks to avoid crashing.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/798
