how to save point cloud and 3d_label as an image using "render_to_image" in GUI?

### 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 all,

First of all, thanks all for this awesome library.
Recently, I am working on open3d GUI as a 3d point cloud processing tool.
But when trying to save the scene as an image (ex. screenshot), as far as I know, there is no such feature to include "3d_label" into the image that I want to save together.

In documentation, the 3d_label is added by the scenewidget class.
And point cloud is added as a geometry by the renderer (Open3DScene) of the scenewidget.
In terms of the screenshot, "render_to_image" function can be used.
But this function is only in the renderer, not scene widget.

Is there a way to save the point cloud and 3d_label togrether as an image ?

Thanks in advance.

Below is the current working environment info.
- Open3D version: 0.15.1
- Python version: 3.7
- OS: Windows / Ubuntu