Update to Rust native webp implementation

[nekowinston]

We should try to make catwalk use https://github.com/image-rs/image-webp instead of relying on libwebp.

[nekowinston]

Current status: it seems like the image-webp implementation as of today has some encoding differences with lossless libwebp.
I'm running into problems when trying some image size & pixel type (rgb vs rgba) combinations, transitioning to webp-pure might take a while.

[nekowinston]

FYI I'm abandoning work on this issue, feel free to pick it up or close as not planned.

[kjnsn]

It looks like using the higher-level image crate could be easier as it provides abstractions over things like pixels