Fix 0 radius arc rendering on png export, canvas

Nathan Leerequested to merge
nathanal/inkscape:partialArcFix692 into master

See #692 (closed)

Issue introduced when optimization was added (directly use elliptical arc, rather than converting it to cubic bezier first).

For reference: https://www.w3.org/TR/SVG11/implnote.html#ArcImplementationNotes

This does not fix other issues (such as path cutting with these arcs)

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