When a face disappears, we seem to get a message from facedetect with
a face count of 0, which we want to just ignore instead of trying to
access face #-1, which causes nasty warnings.
Use generic and unspecififed rgb/caps for now. The exact caps
supported depend on the facedetect element and rsvgoverlay. It's
not clear how this worked before, since facedetect only accepts
24-bit RGB, but the caps advertised 32-bit ARGB/BGRA. In any case,
we don't want to force anything really, so that if any of those
elements acquires support for additional formats we pick those up
automatically.
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
