Flatten Beziers is inaccurate for rotated arcs

This feature is under Extensions -> Modify Path -> Flatten Beziers... and is implemented in flatten.py where it converts all SVG elements to cubic splines and then subdivides them.

It fails to give a good result on the arcs01.svg example from the w3/svg documentation.

As you can see in the image below where I have flattened the arcs and then re-imported the same image for comparison, even the end-points of the arcs don't agree. I would expect the endpoint nodes to always match, even if the curves themselves are approximated.