Every call to BM_faces_join and BM_faces_join_pair has been adjusted to
provide the new face pointer, and a BLI_assert_msg has been added so
that doubles are now consistently identified and flagged as a problem.
BM_faces_join is now also capable of automatically reusing a double
when it is found. This new behavior is currently unused at this point.
Future patch-sets will begin to use it, allowing simplification of
calling functions.
Ref: !137406
702efd6846