how much does scrolling change the camera extrinsics in gui.SceneWidget.scene?

### 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 want to dynamically adapt the point size of a point cloud in the gui.SceneWidget Scene.
I have tried with adding the `gui.MouseEvent.WHEEL.wheel_dy` to the material record point size. However with this linear method, the scrolling changes the point size inconsistently. E.g. after scrolling out a lot and zooming back to the original start position, the points have a different size than in the start.
Is there a way to figure out, how much scrolling effects the camera extrinsics so I can calculate the point size via this parameter to keep consistency?

Many thanks in advance