Add multithreaded ray tracer implementation

include/hittable.h

@@ -0,0 +1,33 @@
+#ifndef HITTABLE_H
+#define HITTABLE_H
+
+#include "ray.h"
+
+class material;
+
+class hit_record {
+  public:
+    point3 p;
+    vec3 normal;
+    double t;
+    shared_ptr<material> mat;
+    bool front_face;
+    void set_face_normal(const ray& r, const vec3& outward_normal){
+             // Sets the hit record normal vector.
+        // NOTE: the parameter `outward_normal` is assumed to have unit length.
+
+        front_face = dot(r.direction(), outward_normal) < 0;
+        normal = front_face ? outward_normal : -outward_normal;
+    }
+    
+};
+
+class hittable {
+  public:
+    virtual ~hittable() = default;
+
+
+    virtual bool hit(const ray& r, interval ray_t, hit_record& rec) const = 0;
+};
+
+#endif