[Cellar] AV1 mapping update

[Steve Lhomme <[e-mail]>]

I updated the AV1 mapping to clean a few sentences.

Since we allow stripping the Sequence Header OBU from the stream when
it's equal to the CodecPrivate one, we need to add it back to the
bitstream for compliance. At least when seeking on keyframes. So I
added a section to explain that.

IMO that's an extra feature of the CodecPrivate that it's meant to be
added to the bistream as-is. And in this case on startup and when
seeking. I wonder if we should add an element next to the CodecPrivate
to describe that. Because in this case it's not entirely opaque to the
demuxer. Or maybe it's implied by the CodecID and is up to the decoder
to use it how it's supposed to be (in this case detecting keyframes
and possibly adding back the Sequence Header OBU).

https://github.com/Matroska-Org/matroska-specification/blob/av1-mappin/codec/av1.md
and the list of changes can be found here
https://github.com/Matroska-Org/matroska-specification/commits/av1-mappin/codec/av1.md

Let me know what you think so we can settle this spec for good.

-- 
Steve Lhomme
Matroska association Chairman