How to save lineset draw to o3d.visualization.draw_geometries(

### 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 need to save lineset drawn onto a point cloud to mark differences. but i am not able to find a function which can help me do this.

looking forward for some guidance,
Thanks in advance.
jyothi