wayland: Compare geometries after chaining up

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 wouldn'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 #150 (closed)

(This should be cherry-picked to gnome-3-28 too)