Blender Git Loki

Temporary fix for gaps in strokes when objects are behind the lines.
Problem report by Light BWK through personal communications, thanks a lot!

Apparently there is something wrong in the way how edges are chained to
create strokes. For some unknown reason, strokes may contain a very small
line segment that proceeds in the opposite direction (e.g., downward
even when adjacent stroke segments proceed upward), resulting in the
reported visual artefact.

This revision is intended to address the reported issue in most cases.
The present solution is not a proper fix of the issue. Another code
update with better understanding of the real cause is due in the future
work.