Function downsample 

pub fn downsample(
    image: &ImageBuffer<Rgba<u8>, Bytes>,
    target: Size<u32>,
) -> ImageBuffer<Rgba<u8>, Vec<u8>>

Available on crate feature image only.

Resamples RGBA pixels down to target.

Premultiplies the image, to avoid fringing the edges of an icon when interpolating transparent pixels.