title: Use custom value for preferredFramesPerSecond
body: <p>I would like to get get my ARkit app running at an specific number of frames per second (25 fps in my case). I can get this by modifying this parameter: <a href="https://developer.apple.com/documentation/quartzcore/cadisplaylink/1648421-preferredframespersecond" rel="nofollow noreferrer">preferredFramesPerSecond</a> but it always sets 30 fps, not 25.
  It works ok for 5, 15, 20, 30, 45 or 60, but why not for 25 fps?</p>
  
  <pre><code>override func viewDidLoad() {
  
      super.viewDidLoad()
      ...
      sceneView.preferredFramesPerSecond = 25
      ...
  }
  </code></pre>
  
  <p>Thanks a lot in advance!</p>