Ambient Viewing Environment metadata

[galad87]

Dolby Vision 8.4 movies recorded by iOS have an additional metadata in the container called "Ambient Viewing Environment", it would be nice to preserve this in Matroska too.

Some info on https://developer.apple.com/documentation/technotes/tn3145-hdr-video-metadata

[robUx4]

Indeed. Not sure if we should use 3 fields or one binary blob with the 3 values. Given we already split the "Content light level information SEI" (D.2.35 in https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-H.265-202407-I!!PDF-E&type=items), we should probably do the same.

[robUx4]

Another reason to split the values: one may not have the x/y specific values from the source (sensor). The global illumination alone is useful. The other fields don't have an "unknown" value.