node_geo_subdivision_surface.cc

This just makes sure that the mesh coming out of the subdivision
surface node has correct normals. Ideally, we would lazily compute
derived data like normals in more cases, but that can be solved later.
Correctness is more important right now.

In order to solve this better, it would be nice if functions like
`BKE_mesh_ensure_normals` would take a `const Mesh *`.
The mesh could be considered to be logically const, because
normals are derived data that is only cached for the current
state of the mesh.