Desktop shell demoted all fullscreen shell surfaces on all active
outputs of a multi-display setup whenever any shell surface was
activated anywhere. This made it impossible to have multiple
fullscreen windows on separate outputs active at the same
time, as creating or activating any shell surface would disable
fullscreen status for all existing fullscreen surfaces.

Make lower_fullscreen_layer() more selective, so on request it
only demotes fullscreen surfaces on a specified weston_output.

The activate() method for a specific surface will now only request
demotion of fullscreen surfaces on the target output of the activated
surface, but leave fullscreen surfaces on unrelated outputs alone.

Desktop wide acting functions like the window switcher or exposay
will still demote all fullscreen surfaces on all outputs to
implement their effect as before.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>