Add BT.2100 HLG OETF as a BuiltinTransform

[nick-shaw]

There is already a built-in CURVE - LINEAR_to_ST-2084 transform, but no HLG curve exists. Only DISPLAY - CIE-XYZ-D65_to_REC.2100-HLG-1000nit which, being display-referred cannot be used for scene light conversions to and from HLG. As per #1968, this curve should be mirrored about zero.

[doug-walker]

@nick-shaw could you please review PR #2029 to make sure the math is what you expect?

(Note that we decided to do it as a FixedFunction rather than a Builtin, but either one should work for you.)

[nick-shaw]

Thanks @doug-walker . I'm not fully au fait with reading C++ code, but as far as I can tell the maths for mirroring the HLG OETF seems correct. In order to support negative values through the HLG EOTF, the OOTF also needs to include mirroring. Is that already the case? I couldn't see a change to that in that PR.