Add multiple spotlights C++

### 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

In C++, what is the best approach to add multiple spot lights? I cannot find an example, but I see that there is a spot light class.

I am trying

open3d::visualization::Visualizer vis;
vis.CreateVisualizerWindow();

but then cannot seem to add lights to the vis object. How is this done?

Thanks!