The raw PDB wrapper _gimp_drawable_get_format() only transfers the
format's encoding, so we need to add the space from the image's color
profile.

Also fix handling of indexed formats: remove our own indexed format
cache and rely on babl_new_palette_with_space() to return the same
format for any (encoding, space) combination.

Also update the PDB docs to reflect that most magic is happening in
the libgimp C wrapper.