Since this code was already written for HDR, and is now per-channel
(because it works better for HDR as well), we can actually reuse this to
get very high quality gamut mapping without clipping. The only required
change is to move the tone mapping from before the gamut map to after
the gamut map. Additonally, we need to also account for changes in the
signal range as a result of applying the CMS when we compute ref_peak,
which is fortunately pretty easy because we only need to consider the
case of primaries mapping to themselves.
Since `HDR` no longer really makes sense as a label, rename it to
`--tone-mapping` in general. Also fits better with
`--tone-mapping-desat` etc.
Arguably we could also rename `--hdr-compute-peak`, but that option is
basically only useful for HDR content anyway because we don't need
information about the signal range for gamut mapping.
This (finally!) gives us reasonably high quality gamut mapping even in
the absence of an ICC profile / 3DLUT.
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