message: ColorType derives Hash
diff: @@ -6,7 +6,7 @@ use buffer::Pixel;
  use traits::Primitive;
  /// An enumeration over supported color types and their bit depths
  -#[derive(Copy, PartialEq, Eq, Debug, Clone)]
  +#[derive(Copy, PartialEq, Eq, Debug, Clone, Hash)]
  pub enum ColorType {
  /// Pixel is grayscale
  Gray(u8),
  