Support --threads option in husk in the render delegate

[jhodgson]

When rendering via husk we'd like to pass the number of render threads through to the render delegate, ideally via hydra or potentially using an environment variable.

[jhodgson]

husk and uses TBB for threading. The -j/--threads option sets the number of cores in the TBB scheduler. We should be able to pick this up using tbb::this_task_arena::max_concurrency().

Since we know when we are running inside husk, we could override options.threads in this case instead of using the value from the render settings.

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-13299