This way we prevent cracks in the model due to discontinuous normals, by using
smooth normals for displacement instead of always getting flat normals after
linear subdivision.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D1916
Not the most memory efficient way to store these things but it's simple and
implementing it better requires some work to natively support subd grids as
a primitive in some way.
It was never fully implemented and will be replaced by OpenSubdiv. Only linear
subdivision remains now. Also includes some refactoring in the split/dice code,
adding a SubdParams struct to pass around parameters more easily.