Fix T81392: subdivision surface poor shading without limit surface

When the limit surface is disabled OpenSubdiv generates a set of linear
patches which are only C0 continuous, not C1. This makes it impossible to
evaluate derivatives at vertices which, in this mode, are by definition put
at boundaries of patches. Normals are calculated from those derivatives.

Solution is to disable normal calculation and let it be done downstream, as
for other modifiers. This limitation is also the reason that non feature
adaptive subdivision is badly suited for GPU evaluation.

Differential Revision: https://developer.blender.org/D9103

Subdivision Surfaces: add boundary smooth option to modifiers

This adds the option to either smooth the entire boundary, or to keep
corners sharp, for the Subdivision Surface and Multiresolution modifiers.

This mainly helps with compatibility with other software. The default
behavior remains to smooth the entire boundary.

Differential Revision: https://developer.blender.org/D8485

Subdivision Surfaces: add option disable using the limit surface

This makes subdivision surfaces compatible with the old subdivision
surface modifier and other applications that do not use the limit surface.

This option is available on the Subdivision Surface modifier.

Differential Revision: https://developer.blender.org/D8413