Is depth from render_to_depth_image() consistent over frames ?

### 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,

Thanks for your work,

I think my question is related to: https://github.com/isl-org/Open3D/issues/2940

I wanted to ask about the depth values from [OffscreenRenderer](http://www.open3d.org/docs/latest/python_api/open3d.visualization.rendering.OffscreenRenderer.html)

That are acquired with: `render_to_depth_image()`.
Are these depth values consistent across frames ?
Meaning that if I use the depth value to reconstruct the 3D points, are those 3D points the same in the next frame ?

Imagine if I do so for all my frames and I collect all 3D points, and render them. Then I should see the global 3D model ?