How to (1:) lock view control (2:) adjust font style for labels

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] 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 have two questions on which I would like some more information or suggestions:

1) I want to create a 2D view mode, in which the user is only able to see the data from a top-view (nadir/bird eye perspective) and is only allowed to zoom and pan. Is there a way I can achieve this? Couldn't find any examples or documentation that helped me in this matter.

2) I noticed some added functionalities ([#3298](https://github.com/isl-org/Open3D/pull/3298)), stating that labels can be displayed in bold and/or italic. I would like to implement this into my code, but I am lacking the knowledge how to do so. Would someone be able to produce an example with the right syntax?

Appreciate any help!

Richard