Thread safety

When using the unsuitable callback, the unsuitable method is passed to C through a global variable. Therefore at least in this case, the code is not thread-safe. It should be thread-safe (not tested though) if -u is not used.

For making the code thread-safe, the unsuitable callback should be made part of TriangulateIO in the C library after checking if this is possible withot mayor modification of triangle.c .

Meanwhile the advise is to put a lock around triangulate calls in multithreaded code.

Start by reading triangle.c and the TriangulateIO definition in the Julia wrapper, focusing on how the unsuitable callback is passed through a global variable and how -u changes that path. Check whether the callback can be moved into TriangulateIO without major changes to triangle.c. Done means the callback path is safe for concurrent triangulate calls, with the -u case and existing lock advice clarified or tested.