func (v Vec3) Dist2DSqr(v1 Vec3) float32 {
	dx := v1[0] - v[0]
	dz := v1[2] - v[2]
	return dx*dx + dz*dz
}
func (v Vec3) Dist2DSqr(v1 Vec3) float32 {
	dx := v1[0] - v[0]
	dz := v1[2] - v[2]
	return dx*dx + dz*dz
}
// 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.