Quite trivial idea -- just pass tread ID to the texture sampling function.
Implemented as a TLS to avoid passing huge amount of extra contexts around.
Should be working on all platforms, but compilation test is required.
Reviewers: juicyfruit, campbellbarton
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D1831
8c113a95e3