fix for fix, ugh!, vertex slide with concave/convex ngons & normal flipping should now work properly in all cases.
... somehow I managed to make test-cases that worked in previous commits but failed in other cases.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54176 954f8c5b-7b00-dc11-b283-0030488c597c