Blender Git Loki

Experimental fix for incorrect view edge visibility.

The problem is that the visibility of view edges in the view map may be incorrect.
The main cause of this issue is that view edges are constructed from a series of FEdges
without testing the visibility of the FEdges being concatenated. Later view edges
are split into pieces if two view edges intersect in the 2D image coordinate system.
After that the visibility of the view edges is computed by taking account of occluding
faces in the 3D scene. In many cases this procedure results in correct line visibility,
but not always (that is the problem).

A simple solution experimentally implemented here is not to chain FEdges at all.
Instead view edges are constructed from at most one FEdge now. This solution is only
applied to sharp FEdges (roughly corresponding to edges in the input mesh data; specifically,
silhouette, crease, border, edge mark, contour, external contour, and material boundary).

A better solution is to reorder the three steps of view edges construction mentioned above,
i.e., first splitting FEdges at 2D intersection, computing the visibility of the FEdges,
then concatenating them to build view edges. This solution is left for future work for now.