In our previous attempts to fix the lightmapper we may have
inadvertently introduced the same issue we were trying to fix. It
appears that rand() will on some platforms introduce a mutex making it
slower and on others may have a per-thread state that would need to be
initialized with srand() on each thread. This slows down the lightbaking
further.
This sets up a separate rng state for each OpenMP thread by calling
rand() only in the single-threaded part of the code. We then keep a
vector of states.
I believe this solves our problems.
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