Fix knife cut bug #33625, failure to cut in otho mode sometimes.
Problem was that the code to limit the front and back planes
for better precision assumed line would have center near origin.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53559 954f8c5b-7b00-dc11-b283-0030488c597c