Blender Git Loki

Distinguish `use_poly_index` flag based on FACES/POLYS type instead of
an explicit argument.

This avoids meaningless cases when setting `use_poly_index` for vertex
or edge BVHs and is nicer for bmesh which does not yet support both
index types (only polys). Eventually BMeshBVHTree could support
tessfaces as well, at which point it could get a similar type enum.