compositor.h: Add shell_interface.get_output_work_area

This will allow plugins to be aware of e.g. panels, to avoid covering
them with other surfaces.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
2 files changed