Morning Nice diagram also.

This should work :
Code:

---

if(mouse_right == 1)		// Pan with right mouse
{
   camera.x -= mouse_force.y * 40 * time_step;
   camera.y += mouse_force.x * 40 * time_step;
}

---

One more thing is stored the camera default position,
if you go too far in the level, you may press SPACEBAR
and return there at anytime.

Code:

---

var camPos;
.....
.....
vec_set(camPos,camera.x);  
while(1)
{
  ......
  ......

  if (key_SPACE)
  {
      vec_set(camera.x,camPos);
  }
}

---