Blender Git Commit Log

Add API to IMB_thumb to enable thread safety.

Indeed, though rather unlucky, we may end up handling same source file from
different preview threads, which could lead to conflicts and bugs.

So idea is to add a simple way of locking a given source file path, since
thumb handling of different paths shall never conflict.

Note that this adds some 'generic' stuff to GHash and Threads area, that are to
be committed separately of course.