title: Using an ARSCNPlaneGeometry to obtain a SCNPhysicsShape produces a sphere shape and not a plane
body: <p>In an application using <code>ARKit</code>, I am using the <code>ARSCNPlaneGeometry</code> class <code>update</code> method to obtain a <code>SCNGeometry</code> from an <code>ARPlaneGeometry</code>. I obtain a <code>SCNPhysicsShape</code> from that geometry to use as the <code>SCNPhysicsBody</code> for a plane node:</p>
  
  <pre><code>func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
      guard let planeAnchor = anchor as? ARPlaneAnchor else { return }
  
      // Plane Geometry
      let planeGeometry = ARSCNPlaneGeometry(device: MTLCreateSystemDefaultDevice()!)
      planeGeometry?.update(from: planeAnchor.geometry)
      let planeNode = SCNNode(geometry: planeGeometry)
  
      // Plane Physics
      planeNode.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(geometry: planeGeometry!))
  
      // Add Node to Scene
      node.addChildNode(planeNode)
  }
  </code></pre>
  
  <p>The problem I have is that even though the plane node displayed is a plane, the physics shape that is generated is not that of a plane but that of a rough sphere. I know this because I am using the scene view debug option <code>.showPhysicsShapes</code>.</p>
  
  <p>I am also continuously updating the geometry and physics shape using the <code>renderer(_:didUpdate:for:)</code> method of the <code>ARSCNViewDelegate</code> and the shape remains the same.</p>
  
  <p>Anybody else run into this problem before?</p>