title: SceneKit: Using pan gesture to move a node that's not at origin
body: <p>I have a SCNNode that I've set at a position of SCNVector3(0,0,-1). The following code moves the node forward or backward along the Z-axis, however, the initial pan gesture moves the node to (0,0,0) and then moves the node in line with the pan gesture.</p>
  
  <pre><code>let currentPositionDepth = CGPoint()
  
  @objc func handlePan(sender: UIPanGestureRecognizer) {    
      let translation = sender.translation(in: sceneView)
      var newPos = CGPoint(x: CGFloat(translation.x), y: CGFloat(translation.y))
      newPos.x += currentPositionDepth.x
      newPos.y += currentPositionDepth.y
  
      node.position.x = Float(newPos.x)
      node.position.z = Float(newPos.y)
  
      if(sender.state == .ended) { currentPositionDepth = newPos }
  }
  </code></pre>
  
  <p>I'd like the node to move from it's set position of (0,0,-1). I've tried setting currentPositionDepth.y to -1, however it does not achieve the desired effect. How can I achieve this?</p>