idiff sometimes locks up while shutting down when the CPU is
oversubscribed. While blender does not rely on the idiff tool
the tests that run on the CI environment do, which causes tests
to occasionally fail due to a timeout.
The root cause is a bit complex but can be found on the oiio tracker
at https://github.com/OpenImageIO/oiio/issues/3851
This change fixes idiff by :
1- Shutting down the thread pool before the main function exits
2- Have the shutdown wait for the pool threads to actually join, to
prevent the OS from forcefully terminating them while they could
potentially still be holding a lock.
7f9021a92f