deinterlace: Add support for alternate fields interlacing
According to https://bugzilla.gnome.org/show_bug.cgi?id=796106
I'm working on adding actual support for this in interlace and deinterlace elements, which rely on this new API/formats.
Is there a patch for deinterlace somewhere? I don't think we should've merged this without at least support in deinterlace, and I was under the impression that deinterlace actually supports this already.