Divide by 3 instead of multiplying by variations of 0.333

Fixes small precision problems.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54559 954f8c5b-7b00-dc11-b283-0030488c597c
Fix "Origin to Center of Mass" not working well with ngons

Now we do simple triangulation and calculate signed area of triangles to
account for concave polygons.

This only works correct for planar polygons but gives better results
overall.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54560 954f8c5b-7b00-dc11-b283-0030488c597c