Blender Git Loki

Fix for thinning strokes at intersections between visible and background hidden lines.

This commit is intended to fully fix the problem described in
https://developer.blender.org/T36425#19 (see also the previous commit rB08528f577dcb).

Addition of a small offset (to avoid singularity in stroke rendering due to overlapping vertices)
was not performed for all overlapping vertices.

Removed the StrokeCleaner and related helper functions which were added as a
temporary workaround in rB2a5b6d9c8f16.