Blender Git Loki

Sculpt: Implement plane deformation falloff for Grab

The plane deformation falloff was introduced in the first version of the
cloth brush, but due to the lack of all the new features and fixes in the
solver it was causing a lot of artifacts for deformation brushes. In
order to avoid that, the cloth brush was always using radial falloff for
the grab brush.

Now the plane falloff is properly implemented using the deformation
constraints.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9320