Add ambient property to Scene

src/scene.cc

@@ -19,6 +19,7 @@
 Scene::Scene()
     : width(640), height(480),
       max_depth(5),
+      ambient(NULL),
       nrays(0),
       pixels(NULL)
 { }

src/scene.h

@@ -50,6 +50,7 @@ private:
     int max_depth;
 
     // Scene objects.
+    AmbientLight *ambient;
     std::list<Shape *> shapes;
     std::list<PointLight *> lights;