* Improve toolpath simplification to avoid blemishes when using Arachne on thin walls.
* Avoid reducing closed perimeters below 3 points.
Downstream code contains assumptions that closed ExtrusionLines are not
degenerate. Open ExtrusionLines already are not reduced below 2 points by
ExtrusionLine::simplify().
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Co-authored-by: SoftFever <softfeverever@gmail.com>