VisualizerWithEditing option to select obscured points

### 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).

### Proposed new feature or change

I would like to add the option to select including obscured points in visualizerWithEditing. This should be easy to implement. In https://github.com/isl-org/Open3D/issues/1482 @prewettg suggested we "comment out lines 374-381 (the section bookended by the glColorMask() calls) it will select all vertices" although im not sure in which file. I guess we could add an if statement instead of commenting out