How to display textures with O3DVisualizer / O3DSceneWidget

### 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 have a TriangleMesh with textures property set with a list of two o3d Images. When I call draw_geometry with the mesh it is displayed correctly with textures in place. However when I use O3DVisualizer / O3DSceneWidget and add the geometry to the scene with a material, the textures are not displayed. Do I have to move the textures into the material? If so, how do I set multiple textures per mesh, is this still possible?