Rework vtkSMPThreadPool of the STDThread SMP backend
This is a rework of the STDThread backend for SMP.
Major changes are:
- SMP calls no longer create threads. A common, global, thread pool is shared between all calls.
- The new backend supports nested SMP calls without spawning additional threads while still having parallelism
This leads to much better performances for small SMP calls and a better scalability in other cases!