Cannot export objects with a specified id

I have a svg document that contains an icon set. For each icon there is a rect with a specific `id` that defines the bounding box of the icon to be exported. With inkscape I can export individual icons as follows:

inkscape --export-filename icon101.png --export-id=icon101 all_icons.svg

Is this possible with CairoSVG? I have noticed that I can add a #fragment to the input file/url, and was hoping I could use this to specify the id of the object to export, but that fails:

```
$ cairosvg -f png -o icon101.png 'all_icons.svg#icon101'

Traceback (most recent call last):
[...]
TypeError: Root node is rect. Should be one of marker, svg, image, or g.
```

Is this use case supported?

Reproduce the CairoSVG CLI command against all_icons.svg, using the #icon101 fragment and the --export-id workflow described in the issue. Trace fragment handling through the export entry point and check how a selected rect becomes the root node. Done means exporting an object by id produces the requested output instead of the root-node TypeError.

Written by the indexing model from the issue text.