use modified vertex coords for calculating display thickness and intersections.

internal improvement to editmesh_bvh.c

- optionally pass cage-coords as an arg, rather then calculating the coords in BKE_bmbvh_new(),
  since all callers already have coords calculated.
- de-duplicate coords creation function from knife and bmbvhm, move into own generic function: BKE_editmesh_vertexCos_get()


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56122 954f8c5b-7b00-dc11-b283-0030488c597c