code: func (gt *GLTriangles) Picture(i int) (pic pixel.Vec, intensity float64) {
  	tx := gt.data[i*gt.vs.Stride()+6]
  	ty := gt.data[i*gt.vs.Stride()+7]
  	intensity = float64(gt.data[i*gt.vs.Stride()+8])
  	return pixel.V(float64(tx), float64(ty)), intensity
  }
docs: // Picture returns the Picture property of the i-th vertex.