Function decode_g4 

pub fn decode_g4(
    input: impl Iterator<Item = u8>,
    width: u16,
    height: Option<u16>,
    line_cb: impl FnMut(&[u16]),
) -> Option<()>

Decode a Group 4 Image

• width is the width of the image.

• The callback line_cb is called for each decoded line. The argument is the list of positions of color change, starting with white.

  If height is specified, at most that many lines will be decoded, otherwise data is decoded until the end-of-block marker (or end of data).

To obtain an iterator over the pixel colors, the pels function is provided.