desktop-shell: add output co-ordinates to get_output_work_area()

get_output_work_area() now returns the absolute work area including the
output's offset.

This will make math a little simpler later when we use it to constrain
window moves.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
1 file changed