lights: make intensity parameter generic.

rtchallenge/src/lights.rs

@@ -28,10 +28,13 @@ impl PointLight {
     /// assert_eq!(light.position, position);
     /// assert_eq!(light.intensity, intensity);
     /// ```
-    pub fn new(position: Tuple, intensity: Color) -> PointLight {
+    pub fn new<C>(position: Tuple, intensity: C) -> PointLight
+    where
+        C: Into<Color>,
+    {
         PointLight {
             position,
-            intensity,
+            intensity: intensity.into(),
         }
     }
 }