title: ScreenToWorldPoint with perspective cameras
body: <p>So I am having an odd problem just now. I wrote a small script that, when atatched to an object will cause it to face the mouse pointer. However, since I switched from an orthographic camera to a perspective camera, the script has ceased to work. I have added in some debug and it looks like the ScreenToWorldPoint is just returning the same value no matter where the mouse is. I suspect this has something to do with the mouse being a fundamentally 2D entity, but I am not sure how to solve the problem.</p>
  
  <p>Any help is much appreciated! :)</p>
  
  <pre><code> void Update () {
       Vector3 difference = camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position;
       //Debug.Log(Camera.main.ScreenToWorldPoint(Input.mousePosition));
       difference.Normalize();
       difference.Set (difference.x, difference.y, 0);
  
  
       transform.up = difference;
  </code></pre>