How to prevent Vert layout expanding when using ImageWidget

### 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

Hello,
I am using an ImageWidget that is child of a Vert layout. When I add an Image to the ImageWidget, the Image is scaled properly to fit the width of the Vert layout but if the Vert layout does not have a sufficient height, it becomes scrollable.
How can I make sure that the ImageWidget is scaled in a way that the Vert layout does not become scrollable (-> both height and width of the parent layout should be considered when the size of the Image is set)?

Thank you in advance!