How to save and restore viewpoint, when using the gui's SceneWidget?

### 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

I want to achieve the same effect with `draw_geometries`'s _ctrl+c_ & _ctrl+v_ functionality in order to take screenshots of a specific geometry under the same camera conditions. The `SceneWidget`'s scene contains a camera object but I haven't figured out a way to use its `copy_from` method, since the Camera class cannot be instantiated. As far as I've seen there is no example in the docs demonstrating this functionality.