The HLG OOTF is defined as a one-parameter family of OOTFs depending on
the display's peak luminance. With the preceding change to OOTF scale
and handling, we no longer have any issues with outputting values in
whatever signal range we need.
So as a result, it's easy for us to support a tunable OOTF which may
(drastically) alter the display brightness. In fact, this is also the
only correct way to do it, because the HLG appearance depends strongly
on the OOTF configuration. For the OOTF, we consult the mastering
display's tagging (via src.sig_peak). For the inverse OOTF, we consult
the output display's target peak.
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