babl: look deeper, and longer for path fishes

babl: look deeper, and longer for path fishes

We now do two iterations of +1 and +2 path lengths that take much longer than
the configured path lengths - and now complain with a debug output listing the
set of conversions taken to satisfy the request - giving hints on what parts
could possibly be shortened to fewer steps.

With this commit the first level of searching deeper also gets a debug message
which would cause GIMP to print a message in some places where it earlier
wouldnt have warned about missing fast paths.