add function BKE_bmbvh_find_face_segment()
given a segment, finds finds an intersecting faces from the first point to the second, needed for checking self intersections (not used yet).


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56104 954f8c5b-7b00-dc11-b283-0030488c597c