MaxCLL is the more authoritative source for the metadata we are
interested in. The use of mastering metadata is sort of a hack anyway,
since there's no clearly-defined relationship between the mastering peak
brightness and the actual content. (Unlike MaxCLL, which is an explicit
relationship)
Also move the parameter fixing to `fix_image_params`
I don't know if the avutil check is strictly necessary but I've included
it anyway to be on the safe side.
Niklas Haas
deb9370779