message: Fix incorrect `get_pixel_[mut_]checked` indexing
  Add missing pixel channel count multiplication to indexing
  Add additional tests to compare methods to `get_pixel`
diff: @@ -735,7 +735,8 @@ where
  let num_channels = <P as Pixel>::CHANNEL_COUNT as usize;
  let i = (y as usize)
  .saturating_mul(self.width as usize)
  - .saturating_add(x as usize);
  + .saturating_add(x as usize)
  + .saturating_mul(num_channels);
  self.data
  .get(i..i + num_channels)
  @@ -910,7 +911,8 @@ where
  let num_channels = <P as Pixel>::CHANNEL_COUNT as usize;
  let i = (y as usize)
  .saturating_mul(self.width as usize)
  - .saturating_add(x as usize);
  + .saturating_add(x as usize)
  + .saturating_mul(num_channels);
  self.data
  .get_mut(i..i + num_channels)
  @@ -1379,14 +1381,22 @@ mod test {
  #[test]
  fn get_pixel_checked() {
  let mut a: RgbImage = ImageBuffer::new(10, 10);
  - {
  - if let Some(b) = a.get_pixel_mut_checked(0, 1) {
  - b[0] = 255;
  - }
  - }
  + a.get_pixel_mut_checked(0, 1).map(|b| b[0] = 255);
  +
  assert_eq!(a.get_pixel_checked(0, 1), Some(&Rgb([255, 0, 0])));
  - assert_eq!(a.get_pixel_checked(100, 0), None);
  - assert_eq!(a.get_pixel_mut_checked(0, 100), None);
  + assert_eq!(a.get_pixel_checked(0, 1).unwrap(), a.get_pixel(0, 1));
  + assert_eq!(a.get_pixel_checked(10, 0), None);
  + assert_eq!(a.get_pixel_checked(0, 10), None);
  + assert_eq!(a.get_pixel_mut_checked(10, 0), None);
  + assert_eq!(a.get_pixel_mut_checked(0, 10), None);
  +
  + // From image/issues/1672
  + const WHITE: Rgb<u8> = Rgb([255_u8, 255, 255]);
  + let mut a = RgbImage::new(2, 1);
  + a.put_pixel(1, 0, WHITE);
  +
  + assert_eq!(a.get_pixel_checked(1, 0), Some(&WHITE));
  + assert_eq!(a.get_pixel_checked(1, 0).unwrap(), a.get_pixel(1, 0));
  }
  #[test]
  