desktop-shell: Always update the shsurf's output to that of the view

In case shsurfs are migrated/moved or started on different outputs other
than the default one, it causes fullscreen views to never being demoted
to a lower stacking level, due to the fact we never update
the view's output whenever that has changed.

Synchronize the desktop shell output's with the view's output in the
transform_handler.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Daniel Stone <daniel.stone@collabora.com>

desktop-shell/shell.c

@@ -3713,6 +3713,8 @@ transform_handler(struct wl_listener *listener, void *data)
 	if (!shsurf)
 		return;
 
+	shell_surface_set_output(shsurf, shsurf->view->output);
+
 	api = shsurf->shell->xwayland_surface_api;
 	if (!api) {
 		api = weston_xwayland_surface_get_api(shsurf->shell->compositor);