Previous code would consider two different face groups sharing no common
edges as fully isolated from each other, and could assign them the same
bitflag facegroup value.
Following FBX recent option to export these bitflags smoothgroups (!135646),
also consider that two different face groups are connected if they only share
common vertices, and assign them different bitflags values.
NOTE: This seems to be the expected behavior in major DCCs actually
using smoothgroups, only considering boundary edges create groups that
generate broken shading when imported by these tools.
NOTE: The 'unique integer identifers' option is kept for OBJ exporter,
as such OBJ files are also found on internet, depending on which app
generated them.
Pull Request: https://projects.blender.org/blender/blender/pulls/135998
b87b58b07e