faceoverlay: fix weird pad creation code

faceoverlay: fix weird pad creation code

The element would create normal pads in its instance_init function,
and then later in NULL->READY create the elements it needs, remove
the pads created in the instance_init function, and add new ghost
pads instead. Not without saving the external peer pads of the old
pads of course, which it would promptly re-link to the new ghost
pads. Do all of that a bit differently.

Fixes the generic/states.check unit test.

https://bugzilla.gnome.org/show_bug.cgi?id=670588