Rec. 2020 is really an SDR spec, but I think HDR10 made it okay to slap
PQ on it, call it an HDR spec. Rec. 2100 came along after and formally
allowed the use of PQ/HLG, so we should use 2100 instead.
For SVT-AV1, rate control is explicitly set via an "rc" option, so set
those options manually for SVT-AV1. CBR doesn't seem to be fully
supported, but it's sort of supported via CVBR (constrained variable
bitrate), which seems to be specific to each GOP size, which is about as
close to CBR as we can get.
