Issue #84: do not process color conversion twice.

Issue #84: do not process color conversion twice.

Until now babl_fish_lut_process_maybe() was always returning 0, even
when a LUT existed, hence a LUT-based conversion happened.

This was bad, first because it was inefficient (the point of the LUT is
that we didn't have to process through usual conversion code paths). But
worse: when the source and destination buffers were the same, we would
end up getting wrong result (since we'd have the source converted
in-place, then re-converted, hence double conversion!).

This was the reason for issue #84 (see screenshot showing very wrong
colors because of double conversion).