- Some are related to shape errors when the allowed error to approximate circle
by segment is large and arc radius small.
- fix the actual error used in ConvertToPolyline().
- Use SHAPE_ARC::DefaultAccuracyForPCB() instead of a fixed value as extra margin
in zones. It should not change something, because it is also a fixed value
(5 micrometers), but it is not a magic number.
-TransformArcToPolygon() fix some issues and add a new algo, based on the arc actual
outline shape (initial algo is still available in code, just in case).