code: func (v Vec3) Dist2DSqr(v1 Vec3) float32 {
  	dx := v1[0] - v[0]
  	dz := v1[2] - v[2]
  	return dx*dx + dz*dz
  }
docs: // Dist2DSqr derives the square of the distance between v and v2 on the
  // xz-plane.
  //
  // The vectors are projected onto the xz-plane, so the y-values are ignored.