message: Add comprehensive test for cymk_to_rgb in decoder.rs
  This test checks all possible colours produced by CYMK (in one channel), and checks that the RGB conversion is correct.
diff: @@ -132,3 +132,23 @@ impl ImageError {
  }
  }
  }
  +
  +#[cfg(test)]
  +mod tests {
  + use super::cmyk_to_rgb;
  +
  + #[test]
  + fn cymk_to_rgb_correct() {
  + // Based on R = 255 * (1-C/255) * (1-K/255)
  + for c in 0..255 {
  + for k in 0..255 {
  + let r = (255.0 - f32::from(c)) * (255.0 - f32::from(k)) / 255.0;
  + let r_u8 = r as u8;
  + let convert_r = cmyk_to_rgb(&[c, 0, 0, k])[0];
  + assert_eq!(convert_r, r_u8,
  + "c = {}, k = {}, cymk_to_rgb[0] = {}, should be {}", c, k, convert_r, r_u8);
  + }
  + }
  + }
  +}
  +
  