Add custom codec callback in avifEncoder

[y-guyon]

No description provided.

[wantehchang]

Yannis: I only took a cursory look at this pull request. The commit message has the subject line "Add custom codec callback in avifEncoder". That is confusing, because these callbacks do not seem to provide access to a custom AV1 codec. Should we change "custom codec" to "custom encode data"?

[y-guyon]

Yannis: I only took a cursory look at this pull request.

Thank you. This PR is a proof-of-concept to kickstart a discussion, not a request to merge code.

The commit message has the subject line "Add custom codec callback in avifEncoder". That is confusing, because these callbacks do not seem to provide access to a custom AV1 codec.

I did not mention AV1. This could be used for any kind of coded samples (AV2?) but to be compliant with the AVIF specification it would have to be AV1 samples indeed.

Should we change "custom codec" to "custom encode data"?

To me the piece of software that generates encoded data is a codec, so "custom codec callback" is accurate. Whether that is used to feed already compressed samples or to actually encode pixels on the spot, it is opaque to the libavif API.

[y-guyon]

Note that avifCodecEncodeImageFunc can use avifEncoderCustomEncodeImageArgs too. It can be implemented in another PR to simplify the review of this PR.

[y-guyon]

I tagged the new avifEncoder fields as "Experimental feature" as there may be overlooked edge cases. I do not think introducing a new EXPERIMENTAL CMake flag is worth it.

The new API can be fuzzed in another PR once the design is settled.

[y-guyon]

I had another go at it to answer Maryla's comments but the added API starts to be large. I wonder if this is still worth it. Thoughts?