desktop-shell: use work area instead of panel size to constrain moves

This fixes the case where an output isn't at y = 0, where the panel height
isn't correct for constraints.

It also kills a bug - moving a window with a mod-drag off the top of the
screen clamped earlier than it should.

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