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

[hardeverick]

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

[kitaharaa]

Hey, any updates on this issue for APIs below version 26?