* WIP: first part of fix - generation of hole cutlines
First problem was that hole cutlines got extended
over the whole length and sometimes lead to coincident
edges which are difficult to resolve for the polygon
cutter.
* Refined solution, fixed#718
- disabled elaborate hole insertion procedure for now as the
performance impact has to be assessed yet and the new scheme
will break many tests
- stricter rejection of ambiguous configurations in the polygon cutter
- fallback is boolean AND now since there is no need to re-invoke the
polygon cutter (we can't do so as we made it more strict).
Performance-wise we replace a merge by an AND step which may even be
faster the output is smaller and the polygon cutter does not need
to be re-invoked.
* Compatibility with other STLs
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