After 20176d03, the Wayland backend only synchronizes with the
compositor after a geometry was set, and it was different from
the current geometry.
That commit was mistakenly comparing the geometry before chaining
up, which would yield a false negative on the case where the
client didn't call set_geometry() before commit().
Fix that by caching the old geometry locally, chain up (and thus
apply the new geometry rectangle), then comparing the old and
current geometry rectangles.
Fixes https://gitlab.gnome.org/GNOME/mutter/issues/150
cf734999fb