Applied patch from Jan Heller that improves logic of go_fishing code

Applied patch from Jan Heller that improves logic of go_fishing code
in babl-fish.c. The current code searches list of all fishes while
looking for suitable preexistent BABL_FISH_PATH instance. The new code
only searches relevant part of database's hash table. Further, the
current code searches for fish path every time such a babl fish is
requested, even though the same fish path has been requested before
and not found. The new code creates dummy BABL_FISH instance with
appropriate source/destination formats and inserts it into the fish
database to indicate that fish path has been searched for and not
found.
* babl/babl-fish.c: (find_fish_path), (find_memcpy_fish),
(babl_fish_get_id), (babl_fish):
* babl/babl-fish-path.c: (babl_fish_path):
* babl/babl-fish-reference.c: (babl_fish_reference):
* babl/babl-fish-simple.c: (babl_fish_simple):
* babl/babl-internal.h:

svn path=/trunk/; revision=304