Re-activated MPI device, and stability fixes
Functionality: Re-activated MPI backend(s) after 0.9 changes to rtcore backends
Stability/Bugfixes:
- anari device now finalizes the frame buffer on bnFrameBufferGetPointer()
- barney now properly rebuilds the optix pipeline if programs get loaded after the first frame
- anari device no longer sometimes missses resetting accumulation buffer
- umeshes reactivated
- various smaller bugfixes throughout