How to update TriangleMesh geometry in 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 can't seem to find a way of updating geometry while using TriangleMesh?
There is `update_geometry` method, but it only can take `Pointcloud` as input, and `add_geometry` can get slow if I only want to update the colors of several vertices...