Always use BablFishPath's for accepted conversions. The path

Always use BablFishPath's for accepted conversions. The path
construction code is the location of the regression test, an
earlier (now unneeded) optimization allowed using the first
and best conversion available. The (not in bugzilla afair) bug
manifest by this depended on the order of .so's on the file system
to manifest itself.
* babl/babl-fish.c: (go_fishing): only accepts paths when we go
fishing for existing conversions.
(babl_fish): elaborated a comment about why we avoid shortcut
conversions and only paths.

svn path=/trunk/; revision=275