libweston: properly orphan wl_output resources

Remove the wl_resource in the head's resource list when we are
removing the wl_output global. We sent global removal events to clients,
the resources should become dummies until clients reap them. Reset user
data so that clients triying to use dummy objects don't hit e.g. a freed
head pointer.

This fixes a theoretical issue: if an enabled output is disabled and
then gets enabled again, mode changes and wl_surface.enter/leave would
still attempt to use the dummy objects. If a client destroyed a dummy
object, we don't have the destructor to remove it from the resource
list, and libweston would hit freed memory.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v5 Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>