Add marbled texture

src/noise.rs

@@ -9,6 +9,9 @@ pub struct NoiseTexture {
 
 impl Texture for NoiseTexture {
     fn value(&self, _u: f64, _v: f64, point: &Point3) -> Color {
-        Color::new(1.0, 1.0, 1.0) * self.noise.default_turbulence(&(*point * self.scale))
+        let sin = self.scale * point.z() + 10.0 * self.noise.default_turbulence(point);
+        Color::new(1.0, 1.0, 1.0)
+            * 0.5
+            * (1.0 + sin.sin())
     }
 }