PolyLinePolygonWithHolesData::iteratorHoles is not LegacyIterator

The class template `PolyLinePolygonWithHolesData`'s member type `iteratorHoles` is not a legal `LegacyIterator` since there is no specialization of `std::iterator_traits` for it.

This leads to the surprising behavior that code won't compile when the iterator is used with the standard library's algorithms, say `std::transform`, and the concept is correctly enforced (for example by compiling with `__GLIBCXX_DEBUG`).

(There might be other iterators for which this is true as well?)