Blender Git Loki

Fix (unreported) meshes changing shading when creating empty clnors data.

When you were using autosmooth to generate some custom normals, and
created empty custom loop normal data, you would go back to an 'all
smooth' shading, cancelling some sharp edges generated by the mesh's
smooth threshold.

Now we will first tag such edges as sharp, such that shading remains the
same. This is not crucial in current master, but it is for clnors
editing gsoc branch!