code: func Alpha(dest *Image, alpha int) {
  	r := dest.Bounds()
  
  	var a, x, y, i int
  	for x = r.Min.X; x < r.Max.X; x++ {
  		for y = r.Min.Y; y < r.Max.Y; y++ {
  			i = dest.PixOffset(x, y)
  			a = int(dest.Pix[i+3])
  			dest.Pix[i+3] = uint8((a * alpha) / 100)
  		}
  	}
  }
docs: // Alpha will modify the alpha channel of the image such that:
  // existingAlpha = existingAlpha * (givenAlpha / 100.0)