code cleanup: bmesh bvh
- remove unused members from BMBVHTree
- move per-raycast vars from BMBVHTree into per-callback structs (weren't threadsafe).
- simplify BKE_bmbvh_find_vert_closest().


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56103 954f8c5b-7b00-dc11-b283-0030488c597c