code: func FlipH(img image.Image) *image.NRGBA {
  	src := newScanner(img)
  	dstW := src.w
  	dstH := src.h
  	rowSize := dstW * 4
  	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
  	parallel(0, dstH, func(ys <-chan int) {
  		for dstY := range ys {
  			i := dstY * dst.Stride
  			srcY := dstY
  			src.scan(0, srcY, src.w, srcY+1, dst.Pix[i:i+rowSize])
  			reverse(dst.Pix[i : i+rowSize])
  		}
  	})
  	return dst
  }
docs: // FlipH flips the image horizontally (from left to right) and returns the transformed image.