///|
/// Horizontal mirror: `dst(x, y) = src(width-1-x, y)`. Each pixel's four
/// channels travel together, so alpha is preserved per pixel.
pub fn Image::flip_horizontal(self : Image) -> Image {
  let out = Image::new(self.width, self.height)
  for y in 0.. Image {
  let out = Image::new(self.width, self.height)
  for y in 0.. Image {
  let out = Image::new(self.height, self.width)
  for y in 0..