Note that this commit completely replaces old behavior of the auto_smooth feature in BI.
Also note that split normals are only handled when no "advanced geometry post-processing" is used
(something like Displace will obviously break it, since it has to re-compute normals after displacement...).
Reviewers: brecht
Reviewed By: brecht
CC: campbellbarton
Differential Revision: https://developer.blender.org/D368
776f8d5a6f