Add Flip_face.

hittable.h

@@ -30,4 +30,24 @@ public:
   virtual bool bounding_box(double t0, double t1, Aabb& output_box) const = 0;
 };
 
+class Flip_face : public Hittable {
+  public:
+    Flip_face(std::shared_ptr<Hittable> p) : ptr(p) {}
+
+  virtual bool hit(const Ray &r, double tmin, double tmax, hit_record &rec) const {
+      if (!ptr->hit(r, tmin, tmax, rec))
+          return false;
+
+      rec.front_face = !rec.front_face;
+      return true;
+  }
+
+  virtual bool bounding_box(double t0, double t1, Aabb& output_box) const {
+      return ptr->bounding_box(t0, t1, output_box);
+  }
+
+  private:
+    std::shared_ptr<Hittable> ptr;
+};
+
 #endif // HITTABLE_H