fix triangulation crash on polygons of size 0

prevent move-only arcs

don't forget lines after last antiooze

Revert "generalize angleBetween" (which only works with -O0)

This reverts commit aaf9b25.

revert nonsense

Update clipper to 6.4.2

don't compare Cairo::RefPtr to int

use clang40 on FreeBSD

Merge branch 'master' of https://github.com/timschmidt/repsnapper

git-update poly2tri lib

(from https://github.com/jhasse/poly2tri)