There was a global 3x3 array of babl fishes used for converting between
blending and compositing pixel representations, these were all hard-coded to
operate within the sRGB babl-space family. This commit updates a per-instance
array during operation prepare instead, that comes preconfigured with fishes
derived from the correct space. Since the same operation instance might get
different space input during its life time we store and compare the cached
fishes with the current format (which is unique depending on space).

This should address the problem seen in issue #2592