Add a lossy vp8 encoder

[tristanphease]

Adds a lossy vp8 encoder. Works to encode images with e.g.

let mut webp_encoder = WebPEncoder::new(&mut buffer);
let mut encoder_params = EncoderParams::default();
encoder_params.use_lossy = true;
encoder_params.lossy_quality = 95;
webp_encoder.set_params(encoder_params);
webp_encoder.encode(
    &res.to_rgb8(),
    res.width(),
    res.height(),
    image_webp::ColorType::Rgb8,
);

Currently:

• Converts an image to yuv and encodes it
• Picks macroblock luma mode / chroma mode based on getting the transformed coefficients and working out what is cheapest
• Has a quality parameter for choosing the quantization of the image

Still more more specific encoding features that could be implemented but perhaps for a future PR.
There's quite a lot of changes and I think there's more cleanup to do, have done some rework of some of the decoder code as well to support using the same objects and functions in both.