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/scene.h

@@ -16,6 +16,7 @@
 #include "basics.h"
 #include "camera.h"
 #include "object.h"
+#include "stats.hh"
 
 
 class AmbientLight;
@@ -70,22 +71,7 @@ private:
     std::list<PointLight *> lights;
 
     // Rendering stats
-    struct Stats
-    {
-        unsigned long TotalRays() const;
-
-        void PrintRayTable() const;
-
-        /* Ray counts */
-        unsigned long primaryRays;
-        unsigned long shadowRays;
-        unsigned long reflectionRays;
-        unsigned long transmissionRays;
-
-    private:
-        void PrintRayRow(const std::string& title,
-                         const unsigned long& value) const;
-    } mStats;
+    charles::Stats mStats;
 
     // Rendering output.
     bool _is_rendered;