How to get rendering Geometry under rendering.Scene

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hi,
I am trying to build a GUI and I want to export the pointclouds that are added to a Scene.
The example Code "[vis_gui.py](https://github.com/isl-org/Open3D/blob/master/examples/python/visualization/vis_gui.py)" demonstrate the way to export an image from a rendering Scene through rendering.Scene.render_to_image (line 771). But somehow I am stucked at the point to get the geometry from a Scene (gui.visualization.rendering.Scene). Can someone help me out? Thank you in advance!