Blender Git Loki

Fix T46015: normals_split_custom_set_from_vertices doesn't work with zero vectors

This was simply broken for vertex case (indexing loop normals with vert indices...).

Turns out to be rather verbose to replace on-the-fly zero normals by default ones correctly,
and do not want to make a full copy of the given custom normals array, so now this one is
editied in place (replacing zero vectors by correct default normals). Don't think this
could be a serious issue anyway.