#81 face normals

[Felice Giovinazzo]

BUG 6 -- Actions performed:

1. Opened Meshlab in debug;
2. Opened bunny70k.ply;
3. Selected Filters-->Normals,Curvatures and Orientation->Recompute Face Normals;
4. Selected Render->Show Face Normals;
5. The face normals are not displayed on the mesh(unlike what happens with the same operation performed on the vertices instead of the faces)