Rewrote a lot of knife tool.  Now allows cut-through
to make new vertices in the middle of faces.
This also fixes knife bugs:
#36678, #35945, #35943, #35387, #35045, #35002.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61144 954f8c5b-7b00-dc11-b283-0030488c597c