Exporting PNG images from O3DVisualizer in higher dpi

### 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 would like to export a PNG based on a current scene using the O3DVisualizer. I call the method:
```python
vis.export_current_image
```

But the resolution is quite bad. Can I set the DPI of the resulting image somewhere?