Blender Git Loki

Tweak to fix for thread concurency in looptri generation.

Even if pointer assignment may be atomic, it does not prevent reordering
and other nifty compiler tricks, we need a memory barrier to ensure not
only that transferring pointer from wip array to final one is atomic,
but also that all previous writing to memory are ?flushed? to
(visible by) all CPUs...

Thanks @sergey for finding the potential (though quite unlikely) issue.