code: func (gt *GLTriangles) Slice(i, j int) pixel.Triangles {
  	return &GLTriangles{
  		vs:     gt.vs.Slice(i, j),
  		data:   gt.data[i*gt.vs.Stride() : j*gt.vs.Stride()],
  		shader: gt.shader,
  	}
  }
docs: // Slice returns a sub-Triangles of this GLTriangles in range [i, j).