We have a bunch of special-casing format passing through the PDB, but
either we were only passing the encoding, or else we were reconstructing
the full format through private intermediate functions. In the
space-invasion world, this is not right. Let's have a proper "format"
type for PDB which does all the relevant data-passing for us, once and
for all!

Note that I am creating a wrapper boxed type GimpBablFormat whose only
goal is to have recognizable GValue since Babl types don't have GType-s.
Moreover I'm not using the GeglParamSpecFormat either, because it just
uses pointers which again are a bit annoying in our various PDB code.
Having a simple boxed arg is better.