The goal is to be able to switch away from OpenMP by default.
In order to achieve this a new parallel_for() function is added,
which follows the same declaration as the one from TBB. For now
the simplest formulation is used where range is provided by start
and last indices (the last one is excluded from the range).
The side effect is that Libmv now expects the threading limits
to be imposed by the default thread area (or have an explicit
thread area with the limit in the caller). In a way this is
similar to other external libraries.
Pull Request: https://projects.blender.org/blender/blender/pulls/136834
ccb493b805