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.