Fix UB in link_holes: iterator stored in realloc-based Array without construction

[psh14]

Problem

link_holes() in src/clipper_tools.cpp stores a ClipperLib::Path::iterator
inside SortingPath, and instances of SortingPath live in a
gdstk::Array<SortingPath>. gdstk::Array grows via realloc()/plain
assignment and never invokes constructors on its slots, so the iterator ends up
copy-assigned into memory that was never constructed.

Under MSVC with _ITERATOR_DEBUG_LEVEL=2 (the default for Debug builds),
std::vector<T>::iterator is not a bare pointer: it carries _Myproxy /
_Mynextiter bookkeeping used to register itself in a linked list owned by its
vector, and operator= reads those fields on the left-hand side before
overwriting them. Reading them out of uninitialized memory is undefined
behavior, and in practice it reads a garbage pointer and crashes with an access
violation the moment gdstk::boolean(...) needs to produce a polygon with a
hole (e.g. Operation::Not where one operand is fully contained in the other).

Release builds (and any /D_ITERATOR_DEBUG_LEVEL=0 configuration) don't show
this, because there std::vector<T>::iterator degenerates to a thin pointer
wrapper with a trivial operator=, so the same UB happens to be harmless —
which is presumably why this has gone unnoticed.

[heitzmann]

Thanks!