code: func (dc *Context) AsMask() *image.Alpha {
  	mask := image.NewAlpha(dc.im.Bounds())
  	draw.Draw(mask, dc.im.Bounds(), dc.im, image.ZP, draw.Src)
  	return mask
  }
docs: // AsMask returns an *image.Alpha representing the alpha channel of this
  // context. This can be useful for advanced clipping operations where you first
  // render the mask geometry and then use it as a mask.