Query TBB for the maximum allowed concurrency, which is free from a bug
in own concurrency detection code. One thing to keep in mind is that now
Cycles is limited by the number of threads in the TBB areana from which
Session is created. This isn't a problem for Blender since we do not limit
arena on Blender side. Could be something to watch out for in other Cycles
integrations.
Differential Revision: https://developer.blender.org/D13658
361702f239