subsurface: unlock cached state on commit if desynced

wl_subsurface::set_desync description states: "If cached state exists when
wl_surface.commit is called in desynchronized mode, the pending state is
added to the cached state, and applied as a whole."

This commit reintroduces an implementation of said behavior, previously
removed in 7daf6da9ac05be2cb74c0983e3caee0b21db75d4.

Strictly speaking, this logic isn't fully correct, as the cached state
and the pending state are applied individually instead, if the cached
state isn't locked by anything else. However, the end result is still
the same.

This commit fixes the issue with Firefox permission popups.

types/wlr_subcompositor.c

@@ -324,6 +324,9 @@ static void subsurface_handle_surface_client_commit(
 		}
 		subsurface->has_cache = true;
 		subsurface->cached_seq = wlr_surface_lock_pending(surface);
+	} else if (subsurface->has_cache) {
+		wlr_surface_unlock_cached(surface, subsurface->cached_seq);
+		subsurface->has_cache = false;
 	}
 }