Webp encode

[fintelia]

The goal of this PR is to have "good enough" pure-Rust webp encoding to enable it by default and remove our dependency on libwebp.

My plan is to completely ignore lossy webp encoding (we already have pure-Rust lossy jpeg and avif encoders) and only focus on lossless mode. Right now, the code is able to produce valid lossless webp's, but they're completely uncompressed. Actually compressing the input will require building huffman trees based on symbol frequency.

I have however implemented hard-coded "subtract green" and "predictor" transforms which should make the data at least somewhat compressible. That's in contrast to libwebp which tries a whole bunch of different transform configurations to see what produces the best compression. Nonetheless, it might be enough to get compression ratios on par with PNG

Update: Huffman compression is implemented.

[fintelia]

Compression ratio on the QOI benchmark suite is currently 29%, compared to about 17% when using libwebp. On the plus side, this encoder is something like 60x faster.

[fintelia]

At this point, the compression speed is about 230M pixels/s, which is right on par with QOI and our fast PNG encoding. And the run length compression got the compression ratio to just under 23%, which is way better than the 27-28% ratios those other encoders get on this corpus. This encoder is coming together better than I was expecting!

[sophie-h]

Would it maybe make sense to move all the webp code to a web-rs crate at some point?

[fintelia]

I started splitting off the webp decoder into its own crate here, but it is proving to be rather involved. I do want to continue with that process, but in the meantime I think it makes sense to go ahead and merge this PR

[hahouari]

I'm not sure if this feature is available or not, the README still mentions Webp encoding requires webp-encoder but this PR is merged 7 months ago. Can someone clearify if the pure-Rust webp encoder is available even if unstable? and if possible the AVIF decoder state as well.

Thank you.

[fintelia]

In the 0.24.9 release, both the pure-Rust webp encoder and the C based encoder are used. The pure-Rust one is used by the high level methods that don't take options, or if you call WebPEncoder::new_lossless. The C-based one is used if you call WebPEncoder::{new, new_with_quality}.

In 0.25.0-preview.0, only pure-Rust encoding is used. Though it actually relies on a copy of this code that has been moved into the image-webp crate.