Using the Knife tool, making cuts that split an edge exactly interpolate
fine for vertex customdata (weights or attributes on the vertex domain)
due to `BM_edge_split` taking care of data layers from the edge and
vertex domain (also mdisps -- which unfortunately dont seem to work
well, but that is for another patch...).
However, making cuts _inside_ a face though dont interpolate at all
(giving default values on new vertices).
With this patch, also interpolate vertex customdata in
`BM_face_split_edgenet`.
Philipp Oeser