This reduces the time needed to get to the first pixel
on screen by multithreading the builtin shader compilation.
We avoid doing this if subprocess compilation is on as
the overhead of potentially partially starting all subprocess
is far greater than the benefit of paralllel compilation.
For some reasons, the compilation is much slower when
done async for these shaders (on Metal ~200ms > ~1.2ms),
so the saving might not be substantial.
Mac M1: First frame 6s > 5s.
Pull Request: https://projects.blender.org/blender/blender/pulls/139627
4c655f076c