Implement motion blur

ray.h

@@ -7,12 +7,15 @@ class ray {
     private: 
         point3 orig;
         vec3 dir;
+        double tm;
 
     public: 
         ray() {}
 
         ray(const point3& origin, const vec3& direction) : orig(origin), dir(direction) {}
         
+        ray(const point3& origin, const vec3& direction, double time) : orig(origin), dir(direction), tm(time) {}
+
         const point3& origin() const {
             return orig;
         }
@@ -21,6 +24,10 @@ class ray {
             return dir;
         }
 
+        double time() const {
+            return tm;
+        }
+
         point3 at(double t) const {
             return orig + t * dir;
         }