Add Camera::LookAt

This is identical to the POV-Ray look_at keyword for cameras. In fact, I stole the code almost directly from them… It's super handy for panning and tilting the camera towards a particular point, and saves me having to do dubiously accurate direction vector calculations by hand.

I still need to figure out what exactly this math is doing.

src/camera.h

@@ -35,6 +35,8 @@ struct Camera
     const Vector3& GetUp() const;
     void SetUp(const Vector3& up);
 
+    void LookAt(const Vector3& pt);
+
     virtual Ray compute_primary_ray(const int x, const int width,
                                     const int y, const int height) const = 0;