Blender Git Loki

Experimental fix for degenerate triangles in imported mesh data.

This commit is an attempt to address degenerate triangles (i.e.,
triangles whose area is zero) that cause incorrect line visibility in
Freestyle.

There are two forms of degenerate triangles. Let A, B and P denote
the three vertices of a triangle. A degenerate triangle is a triangle
such that 1) A and B are in the same position in the 3D space, or
2) the distance between point P and line segment AB is zero. Note
that the first form is a special case of the second form. Degenerate
triangles in the first form is easy to remove by the Remove Doubles
command. This commit is intended to address those degenerate triangles
in the second form.

The implemented fix cannot address degenerate triangles in general.
It fails when a triangle touches with multiple degenerate triangles.
A more general solution needs to be implemented.