Blender Git Loki

Fix T39735: New auto smooth creates artifacts with flat shaded faces(BI)

This actually had nothing specific to new split normals, it was an internal limitation
of BI raytracer, which would check against neighbor face shadowing only when they shared
a common vertex, now it also performs checks when both faces have a vertex with a common
"ancestor" (org index).

Note this allows to also fix same issue when using SplitEdges modifier (and potentially
others?), but only when AutoSmooth is enabled (due to some compute/mem overhead, we
do not want to enable this code systematically).

Thanks to Brecht for advices and review!