reduce clipper offset miter limits from 3 to 2 in light of thinwall work

Hopefully medial axis thinwall and thin gap work will allow us to
avoid the tradeoffs involved in higher/lower miter limit values.
3 gives some spikes, but merges gaps; 1 reduces spikes but leaves gaps
Medial axis thinwall work should fill in/link those gaps.
reverse some contours so thin point fragments come at end of path
junction trim - trim medial axis fragment ends that overlap other frags
fix some junction trim bugs that caused too much end clipping