Freestyle SVG export: handle relative paths

Previously, relative paths were not handled well. The result was an
error in the console and no output at the expected location.

The path resolving order is as follows:

* use the given path (scene.render.frame_path()) when it is absolute
* otherwise, try to join with the current blendfile's location
* otherwise, try to join with the folder from which blender was called

Tested on linux, but not on Windows