Sometimes when the end caps pointed strait up (z axis) their faces would
become inverted.
It seems like the code tried to rely on a certain vertex order to always
happen. However this edge case did manage to produce an order that would
produce inverted cap faces. So now I introduce a normal check instead
so we can be certain that the face will have the normal direction we
want.
Reviewed By: Jacques Lucke
Differential Revision: http://developer.blender.org/D4138
c83848722a