magnifier: Use the new get_client_content_rect API

magnifier: Use the new get_client_content_rect API

As we have removed is_client_decorated from Mutter in the last release.
Instead of re-adding it and re-exposing SSD/CSD differences and making
gnome-shell deal with that, we created a new API that does exactly what
is_client_decorated was used for, so switch to using that.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7984

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3522>
(cherry picked from commit d92f8daf62ab5a9d1c02f82701d74a9f87ae01ee)