Get and set the perspective

### 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'd like to learn is there anyway to get/set the perspective (of the camera) in `Visualizer` like the functionalities released in `o3d.visualization.gui`? I am working on reading multiple frames of point-cloud, so it would be great if I can maintain the perspective. Then the user does not need to adjust the camera every time the new frame comes in.

Thanks!