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Sculpt: Use plasticity for softbody influence

Previously the softbody strength property was controlling the strength
of the constraints that pin all vertices to the original location. This
was causing problems when the forces were trying to deform the vertices
too much, like when using gravity or grab brushes.

Now softbody is implemented with plasticity, which creates constraints to
a separate coordinates array. These coordinates are deformed with the
simulation, and the plasticity parameter controls how much the
simulation moves the coordinates (plasticity 0), or the coordinates move
the simulation back to its previous position (plasticity 1).

This creates much better and predictable results and adding softbody
plasticity to the brushes can increase its control and the stability of
the simulation.

Reviewed By: sergey, zeddb

Differential Revision: https://developer.blender.org/D9187