Union: contour order dependent on platform

[madig]

Original issue: typemytype/booleanOperations#55 -- reproducer script: https://gist.github.com/madig/998ec05ef8a496f984aaa86aeceda6fc

This passes on my Windows 10 machine and on the one of my colleague, but fails on macOS and Linux, which order the contours differently. Linux and macOS produce more similar output, but do not come to the same result.

Maybe there's some undefined sorting going on.