code: func (im *Image) Set(x, y int, c color.Color) {
  	if !(image.Point{x, y}.In(im.Rect)) {
  		return
  	}
  
  	i := im.PixOffset(x, y)
  	cc := BGRAModel.Convert(c).(BGRA)
  	im.Pix[i] = cc.B
  	im.Pix[i+1] = cc.G
  	im.Pix[i+2] = cc.R
  	im.Pix[i+3] = cc.A
  }
docs: // Set satisfies the draw.Image interface by allowing the color of a pixel
  // at (x, y) to be changed.