Add the ability to get an image of the current view through sceneView.snapshot()

For my use case I need to grab an image of the current view, but the `sceneView` is private and inaccessible. I don't think `sceneView` needs to be made public, but at least offer new `snapshot()` method that will give you the current view of the scene.
I might submit a PR for this if necessary.

Thanks
