func (v Vec3) Dot2D(u Vec3) float32 {
	return v[0]*u[0] + v[2]*u[2]
}
func (v Vec3) Dot2D(u Vec3) float32 {
	return v[0]*u[0] + v[2]*u[2]
}
// Dot2D derives the dot product of two vectors on the xz-plane. u . v
//
// The vectors are projected onto the xz-plane, so the y-values are ignored.