message: modified map_without_alpha to use apply_with_alpha
diff: @@ -179,7 +179,7 @@ impl<T: Primitive + 'static> Pixel for $ident<T> {
  F: FnMut(Self::Subpixel) -> Self::Subpixel,
  {
  let mut this = (*self).clone();
  - this.apply_without_alpha(f);
  + this.apply_with_alpha(f, |x| x);
  this
  }
  