message: Implement ColorType::has_alpha
  Since the enumeration is non-exhaustive, implementing this method might
  be more brittle than necessary. This commits to having such a judgement
  available for all future color types but that should not be problematic.
diff: @@ -48,6 +48,16 @@ impl ColorType {
  }
  }
  + /// Returns if there is an alpha channel.
  + pub fn has_alpha(self) -> bool {
  + use ColorType::*;
  + match self {
  + L8 | L16 | Rgb8 | Bgr8 | Rgb16 => false,
  + La8 | Rgba8 | Bgra8 | La16 | Rgba16 => true,
  + __NonExhaustive(marker) => match marker._private {},
  + }
  + }
  +
  /// Returns the number of bits contained in a pixel of `ColorType` ```c``` (which will always be
  /// a multiple of 8).
  pub fn bits_per_pixel(self) -> u16 {
  