Function encode_lossless 

pub fn encode_lossless(
    image: &ImageBuffer,
    optimize: usize,
    exif: Option<&[u8]>,
) -> Result<Vec<u8>, EncoderError>

Encodes an image as a still lossless WebP container.

optimize must be in 0..=9.

If exif is present it is embedded as a raw EXIF chunk.