message: Fix PBM tests
  Note that the colors are decoded as L8 which should map white to the
  full brightness 255 and not to dark gray, 1. However some part of the
  decoder seems to be broken in this regard.
diff: @@ -1096,7 +1096,7 @@ ENDHDR
  let mut image = vec![0; decoder.total_bytes() as usize];
  decoder.read_image(&mut image).unwrap();
  - assert_eq!(image, vec![1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0]);
  + assert_eq!(image, vec![255, 0, 0, 255, 0, 0, 0, 255, 0, 0, 255, 0]);
  match PnmDecoder::new(&pbmbinary[..]).unwrap().into_inner() {
  (
  _,
  @@ -1128,7 +1128,7 @@ ENDHDR
  let mut image = vec![0; decoder.total_bytes() as usize];
  decoder.read_image(&mut image).unwrap();
  - assert_eq!(image, vec![1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0]);
  + assert_eq!(image, vec![255, 0, 0, 255, 0, 0, 0, 255, 0, 0, 255, 0]);
  match PnmDecoder::new(&pbmbinary[..]).unwrap().into_inner() {
  (
  _,
  