impl Intersectable for Plane {
    fn intersect(&self, ray: &Ray) -> Option<f64> {
        let normal = &self.normal;
        let denom = normal.dot(&ray.direction);
        if denom > 1e-6 {
            let v = self.origin - ray.origin;
            let distance = v.dot(&normal) / denom;
            if distance >= 0.0 {
                return Some(distance);
            }
        }
        None
    }
}