optimized fishing when looking up existing fish (should perhaps be moved

optimized fishing when looking up existing fish (should perhaps be moved

* babl/babl-fish.c: (go_fishing): optimized fishing when looking
up existing fish (should perhaps be moved to lists going from given
formats, similar to how conversions are added there instead.) Made
go_fishing accept BABL_FISH_REFERENCE when source and desintation
formats are equal.
(babl_fish_process): removed most of the need for BablImage for
linear buffers, do a memcpy when source and destination formats
are equal (and we're a BABL_FISH_REFERENCE).
* babl/babl-db.[ch]: moved the BablDb struct out into public to
allow faster iteration through it.

svn path=/trunk/; revision=250