According to reports and testing in blender/blender-addons#104434, DCCs
tend to behave badly when rebuilding normals from 'bitflags'
smoothgroups, if different smoothgroups using the same bitflag value share
(are connected by) some common vertices, even if there are no common
edges between them.
This commit adds a new option to the RNA API generating smooth groups,
to also consider smooth groups only sharing vertices as neighbors.
It also makes related required change to implementation, and some
refactor of the API, splitting public functions between 'normal' and
'bitflags' versions.
This should make changes proposed in blender/blender-addons#105516
much simpler, and allow for a matching behavior in smoothgroups
generated by the OBJ exporter as well.
Pull Request: https://projects.blender.org/blender/blender/pulls/135248
00de07f752