Blender Git Loki

IMB_thumb: add thread locking API.

General idea is that, if several threads are handling thumbnails at the same time,
they can end working on the same file at some point, which will generate conflict.

To avoid this, threads can now lock a given filepath.

Note that locking data is allocated on a ref-count basis, to avoid keeping the GSet
in memory when not needed. Also, we are using global LOCK_IMAGE mutex for now.

Needed for upcomming filebrowser rework.