Set chroma_sample_position only for YUV 4:2:0

[wantehchang]

The chroma_sample_position syntax element is only used for YUV 4:2:0 in
the AV1 bitstream. So set chroma_sample_position to
ChromaSamplePosition::Unknown for all other YUV formats, including
monochrome (YUV 4:0:0).

Note: For still images, the most common chroma sample position in
practice is the "center" position. Unfortunately the AV1 specification
does not have a value for the "center" chroma sample position. See
AOMediaCodec/av1-avif#88 and
AOMediaCodec/av1-spec#308. This pull request
preserves the ChromaSamplePosition::Colocated value that is currently
used, but it is likely to be incorrect when YUV 4:2:0 is supported.

[wantehchang]

@kornelski Please review. Thanks!

[kornelski]

Thanks.

I'm not planning to support chroma subsampling of color channels ever. If I remember correctly, this chroma setting was only a workaround for lack of monochrome encoding.

[wantehchang]

I see. I guess older versions of rav1e could not encode monochrome (YUV 4:0:0) images. The current version of rav1e encodes alpha images as monochrome correctly.

The current cavif-rs code uses only ChromaSampling::Cs444 and ChromaSampling::Cs400. If it will stay this way, we can simply set chroma_sample_position to ChromaSamplePosition::Unknown unconditionally. Would you like me to do that?

[kornelski]

Yes, please