The clay strips brush tip tests distances against a cube with beveled Z
aligned edges. This cube was positioned with its center in the surface
of the mesh, so when producing large deformation, some vertices that
should be deformed were positioned further than the cube's Z dimension,
so they were left behind, producing artifacts.
This displaces and deforms the local space to position the brush tip
cube (now a prism) towards the deformation direction, so more vertices
can be included, removing most of these artifacts.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D8004
560a73610b