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>