babl: progressively increase search depth from 2 to 4

babl: progressively increase search depth from 2 to 4

Follow-on from previous commit, the specific paths looked for
that are longer than 2 involving formats with spaces are 3
steps long. Doing a 4 step deep search in these common cases
still introduces noticable "warm-up" jankiness in GIMP.

To additionally ensure we're not missing used valid and faster
than reference conversions in GIMP-2.10 we do an additional
search at depth 5 when no converison had been found *and* the
destination format has a space differing from sRGB.

asdf