This is similar to an ExtrusionLoop, but it is open.
It may contain multiple chained paths with differing parameters.
This allows one to have a hierarchy of paths, where the ExtrusionEntityCollection
will be chained by the G-code generator, but ExtrusionMultiPath will not.
Added a safe variant of offset(const Slic3r::ExPolygon...), which offsets each loop separately.
New functions "remove_sticks" to remove zero area parts of polygons.
New functions "remove_small" and "remove_degenerate" for polygon clean up.
Extended the C++ supports, those are not finalized yet though.
