Internal error on triangulation

[klayoutmatthias]

For the test case and discussion see here: https://www.klayout.de/forum/discussion/2883/pex-error-src-db-db-dbplc-cc-543-false

[klayoutmatthias]

Referencing #2429 as this ticket was created with the same intention.

[klayoutmatthias]

Closed as duplicate of #2329