dispose vrHelper when scene is disposed

src/Cameras/VR/babylon.vrExperienceHelper.ts

@@ -310,6 +310,10 @@ module BABYLON {
             scene.getEngine().onVRRequestPresentComplete.add(this._onVRRequestPresentComplete);
             window.addEventListener('vrdisplaypresentchange', this._onVrDisplayPresentChange);
 
+            scene.onDisposeObservable.add(()=>{
+                this.dispose();
+            })
+
             // Gamepad connection events
             this._webVRCamera.onControllerMeshLoadedObservable.add((webVRController) => this._onDefaultMeshLoaded(webVRController));
             this._scene.gamepadManager.onGamepadConnectedObservable.add((pad) => this._onNewGamepadConnected(pad));