Bug with non-square images

[FreddyWordingham]

Noting a known bug when rendering non-square images:

I think this is probably in the util.cols_to_image function (I've probably presumed something incorrectly about the png format)

pub fn cols_to_image(arr: &Array3<u8>) -> RgbImage {
    let (width, height, _) = arr.dim();
    RgbImage::from_vec(
        width as u32,
        height as u32,
        arr.as_slice().unwrap().to_vec(),
    )
    .expect("container should have the right size for the image dimensions")
}