core/colorspace: implement support for the HLG transfer function

Xaver Huglrequested to merge
work/zamundaaa/hlg into master

It's used in some videos, and Firefox needs it to be supported by the compositor in order to enable HDR by default.

The spec for the EOTF is at https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2100-3-202502-I!!PDF-E.pdf, page 6. For the OOTF I calculated alpha so that 1.0 maps to the maximum luminance.

BUG: 508817

Edited by Xaver Hugl

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