code: func (dc *Context) InvertMask() {
  	if dc.mask == nil {
  		dc.mask = image.NewAlpha(dc.im.Bounds())
  	} else {
  		for i, a := range dc.mask.Pix {
  			dc.mask.Pix[i] = 255 - a
  		}
  	}
  }
docs: // InvertMask inverts the alpha values in the current clipping mask such that
  // a fully transparent region becomes fully opaque and vice versa.