Blender Git Loki

Cycles: Fix possible race condition when generating Beckmann table

Two issues here:

- Checking table size to be non-zero is not a proper way to go here. This is
because we first resize the table and then fill it in. So it was possible that
non-initialized table was used.

Trickery with using temporary memory and then doing table.swap() might work,
but we can not guarantee that table size will be set after the data pointer.

- Mutex guard was useless, because every thread was using own mutex. Need to
make mutex guard static so all threads are using same mutex.