utility vector functions for flipping one vector about another:
  nicer then interp_v3_v3v3(v, v1, v2, -1.0f);


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53898 954f8c5b-7b00-dc11-b283-0030488c597c