* Consider non-manifold edges as sharp, as in split normals handling.
* Consider edges from sharp polys as sharp!!!
* Fix returned number of groups (was off-by-one for non-bitflags grouping, could also
be wrong in case of id overflow).
Note about using sharp polys too to define groups: Only current use of this function
(Obj exporter) does not need that, because it does its own check for sharp faces.
However, we might reuse that func in other places in future (e.g. in custom split
normals area), so better to get a consistent behavior!
