Rough draft including ravif for AVIF encoding

[HeroicKatora]

Converts the data to the Img buffer and encodes it into an intermediate vector, then writes this into the output writer. It tries to avoid actually allocating for that conversion. It considers that RGBA8 can be converted from any byte slice and uses the applicable conversion. Any non-rgba8 image is converted using an intermediate buffer. (cc: @fintelia) Some of this code may be used for the conversion feature as well.
However it does not support limits.

Closes: #1152

[HeroicKatora]

I have no idea how to test the encoding without depending on the availability of libavif/avifdec on the test system. For now, I've validated the images manually and it appears to work.. also note that ravif is of course tested itself.

[kornelski]

For testing:

• to test just the MP4/HEIF structure, I wrote https://lib.rs/avif-parse It's pure Rust, cut-down fork of Firefox's mp4parse. It's pretty mature.

• to test pixels decoded from AV1, I wrote https://lib.rs/aom-decode It needs cmake and builds libaom, so it's a relatively heavy dependency, and that crate is still in flux. Sadly, there's no pure-Rust AV1 decoder.

[HeroicKatora]

@kornelski The pixel decoding would be more interesting, of course then also with full bindings, and the pure structure seems like a duplicated effort. I suppose we just wait here, any updates on the situation are appreciated.

[est31]

There is a partial rewrite of dav1d in https://github.com/rainliu/rav1d/ but it yielded an error no matter which image I gave it.