Move Stats to its own module

Move Stats to its own module and pass it around to the intersection methods to keep track of how many tests and successful intersections there are.

src/object_sphere.cc

@@ -66,9 +66,12 @@ Sphere::set_radius(float r)
  */
 bool
 Sphere::DoesIntersect(const Ray& ray,
-                      TVector& t)
+                      TVector& t,
+                      Stats& stats)
     const
 {
+    stats.sphereIntersectionTests++;
+
     /* Origin of the vector in object space. */
     Vector3 rayOriginObj = ray.origin - GetOrigin();
 
@@ -119,6 +122,7 @@ Sphere::DoesIntersect(const Ray& ray,
         }
     }
 
+    stats.sphereIntersections++;
     return true;
 }