WebP output

[fallaciousreasoning]

In #14 I updated wasm-thumbnail to output a PNG file, if the input file has transparency, as we were breaking Logos (and any other images with transparency). We don't output to a PNG when we have no transparency, as the quality/size tradeoff for photographs is much better in a JPEG than a PNG

@fmarier pointed out it would be ideal if the output format was unrelated to the original input image, and that we can solve two birds with one stone if we just use WebP. Unfortunately, the Rust image crate we're using doesn't have native WebP encoding support, and depends on the C based libwebp library.

I think it should be possible to compile that library in to our WASM binary (though I'm pretty hazy on the details). Maybe this is something we should look into?

(this seems strongly related to #7)