Fixed the issue where the new pop-up box cannot capture mouse events after switching scenes.

[hpyou]

this.captureOnPointerEnter only has a setter method, and its value is always undefined, causing stopCaptureOnEnter() to fail to be called, and the module level variable _scene can not changed to current scene when switch scenes. fix issue https://github.com/BabylonJS/Extensions/issues/277

[sebavan]

@ericwood73 I think it would be great if you can have a look at this one ?