Add support for different renderers

[benoitkugler]

Hi !
This is not an issue, but rather a proposal; feel free to close it if not interested !

I would like to use oksvg to render svg files to an other target than an image: I have in mind a PDF file, but it could be any other target.

To do so, I propose an intermediate package which handles the svg parsing but do not import rasterx (or any other backend package). Instead it delegates the drawing to a Driver interface.

This interface is then easily implemented by a thin wrapper around rasterx, but may also be implemented by a PDF writter.

It's true that a PDF writer could directly implement the Scanner interface, but it would be rather inefficient since PDF directly supports high-level path painting operators.

I have a proof of concept at https://github.com/benoitkugler/oksvg : the raster backend is complete, and the PDF one is
in progress, but already support a large number of features.

If you considerer this idea interesting, we can discuss the details of a possible PR...