Blender Git Loki

BLI_bvhtree_overlap: Improve non-multithreaded usage

Although not using multithreading, the `thread` parameter in the
`BVHTree_OverlapCallback` callback always returned a value between the
"number of threads".

This parameter should always be 0 in such cases.

Also a `BLI_Stack` was created for each "thread" and used.
This small overhead is no longer seen.

Differential Revision: https://developer.blender.org/D6510