libweston-desktop/xdg-shell: Check window geometry instead of surface size against...

libweston-desktop/xdg-shell: Check window geometry instead of surface size against configured size

Shell surfaces may have a geometry that is different to the size of
their main surface, e.g. due to subcompositing.

In states where size is strictly enforced (fullscreen and maximized),
the size that the compositor wants must be checked against the window
geometry and not just the main surface size.

Fix by calling weston_desktop_surface_get_geometry and using that size
instead of main surface size.

Signed-off-by: Philipp Kerling <pkerling@casix.org>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>