func Vec3Dist2DSqr(v1, v2 Vec3) float32 {
	dx := v1[0] - v2[0]
	dz := v1[2] - v2[2]
	return dx*dx + dz*dz
}
func Vec3Dist2DSqr(v1, v2 Vec3) float32 {
	dx := v1[0] - v2[0]
	dz := v1[2] - v2[2]
	return dx*dx + dz*dz
}
// Vec3Dist2DSqr derives the square of the distance between v1 and v2 on the
// xz-plane.
//
// The vectors are projected onto the xz-plane, so the y-values are ignored.