renderer/native: Clear old KMS updates on views rebuild

If there are any pending updates, for example if we painted one of
multiple monitors but without having posted the update due to waiting
for another monitor to be painted, but before we paint all of them and
post the update, another hotplug event happens, we'd have stale pending
KMS update. When that update eventually would be processed, we'd try to
apply out-of-date updates which may contain freed memory.

Fix this by discarding any update when we're rebuilding the views. We
can be sure not to need any of the old updates since we're rebuilding
the whole content anyway.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1928
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2216>


(cherry picked from commit 8e91c629)