Support outputting higher bit-depth AVIF

[mertalev]

Feature request

What are you trying to achieve?

As a photo library, we convert and sanitize images to WebP and JPEG for performance. But these formats can compromise on color fidelity since they're limited to 8 bits. As a result, it's not uncommon for users to want to avoid using the converted images altogether.

AVIF, unlike these formats, allows for higher bit depths of 10 and 12. Combined with its higher compression, this helps bridge the gap between formats like WebP and the often very large originals.

AVIF also supports HDR, but this is out of scope for this request since it likely requires upstream changes in libvips.

When you searched for similar feature requests, what did you find that might be related?

#3358 changed AVIF to always have 8-bit output for compatibility with AOM builds. Defaulting to 8 bits would still maintain this behavior, but allow for higher bit depths when the API consumer knows their build supports this.

What would you expect the API to look like?

Similar to the TIFF output options, a bitdepth setting that defaults to 8 and can be configured to 10 or 12.

What alternatives have you considered?

P3 helps, but ideally it'd be complementary to this rather than an alternative.

Ultra HDR is a way to embed gain maps into JPEG. However, it's still limited in color fidelity and doesn't benefit from AVIF's better compression. libvips also doesn't currently support Ultra HDR, so it'd be quite a bit more work.

[mertalev]

Oh, I just saw that libvips already supports HDR for AVIF and even has the same bitdepth parameter with options of 8, 10 or 12. That's exciting!