Implement the SVG path syntax to read back the strings
that we generate when serializing paths. The tests for
this code are taken from librsvg.
This includes an elliptical arc implementation according
to the SVG spec. The code is mostly taken from librsvg,
but pretty directly follows the SVG spec implementation
notes. We don't export this, since the parametrization
is inconvenient. We do want an arc_to API, but
these are not the arcs we are looking for.
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