Partially revert [0] which replaced BMBVHTree with SnapObjectContext.
While SnapObjectContext is a comprehensive method of performing
ray-casts, the purpose of edge-slide visibility checks is mainly to
exclude back-facing vertices.
Using SnapObjectContext has some down-sides that include iterating
over all dupli-instances for every vertex, details in code-comments.
Resolve be restoring simpler self-occlusion check.
[0]: 2d50a41d77
Ref !128016
a13513ab25