Mantaflow: call MANTA::terminateMantaflow on exit

terminateMantaflow was never called, this leak is more of a technicality
since it's only called on exit.

Also make Py_Initialize/Py_Finalize optional in Pd:setup/finalize
as it caused Blender to crash, finalizing Python twice.

Add a patch to extern/mantaflow to keep track of changes in Blender
from up-stream.