color: add min/max absolute luminance
This MR depends !33 (merged).
This completes the SMPTE ST 2086 HDR static metadata in the image description protocol by adding the min and max absolute luminance.
Because these are only used with BT.2100 PQ, the request to set them is restricted to that. Likewise, compositors should send the event only when they actually work in absolute luminance units and probably drive a display in BT.2100 PQ mode. Even then, we know the luminance values are probably not the actual physically emitted luminances.
I see that CTA-861-H also defines MaxFALL and MaxCLL but I'm not sure they belong here...