This currently only works when using lcms-based color management
(--icc-profile-*).
In principle, we could also support using lcms even when the user has
not specified an ICC profile, by generating the profile against a fixed
reference (--target-prim/--target-trc) instead. I still might do that
some day, simply because 3dlut provides a higher quality conversion than
our simple gamut mapping does for stuff like BT.2020, and also because
it's now needed to enable embedded ICC profiles. But that would be a
separate change, so preserve the status quo for now.
(Besides, my opinion is still that you should be using an ICC profile if
you care about colors being accurate _at all_)
Niklas Haas
fee6b287a5