[TransformControls] Don't attempt to capture pointer if pointerlock is active

[jbaicoianu]

Attempting to interact with TransformControls when pointerlock is already active in your app throws the following error:

Uncaught DOMException: Failed to execute 'setPointerCapture' on 'Element': InvalidStateError at TransformControls.onPointerDown

This patch skips the attempt to call this.domElement.setPointerCapture( event.pointerId ) if pointerlock is already active

[mrdoob]

Thanks!