How to set orthographic projection in camera

### 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 have an orthographic projection view in my gui and I can't get it to work. There is a way according to the doc to setup orthographic projection but I can't get it to work right.

The object is stretched in the view however. In addition I can't zoom in or out. Is there any way to get a "normal" orthographic perspective?

Currently I use this code to setup orthographic projection:

```
self._scene.setup_camera(0, bounds, bounds.get_center())
proj = rendering.Camera.Projection.Ortho
mins = bounds.get_min_bound()
maxs = bounds.get_max_bound()
self._scene.scene.camera.set_projection(
proj,
mins[0],
maxs[0],
mins[1],
maxs[1],
mins[2],
maxs[2]
)
```