How to slow down the camera movement speed in fly mode?

### 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'm using the SceneWidget in Python and I have my point clouds and everything loading just fine. I have set_view_controls(SceneWidget.FLY) in there so I can fly around and through the point clouds.

However, the WASD controls move the camera way too fast through the scene, making it difficult to look at something with very much precision. I end up flying past things, or I have to touch the keys as quickly and/or lightly as possible (sometimes it's not possible) to get to where I want to be. I've tried scaling the entire scene by 100 but that didn't do anything about the camera movement. My point clouds are already quite large outdoor environments anyway.

Is there any way to slow down the camera movement speed in fly mode?