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Sculpt: Multires Displacement Smear

This tool implements smearing for multires displacement over the limit
surface, similar to how smearing for colors and topology slide works.
When used the displacement values of the vertices "slide" over the
topology, creating the effect of smearing the surface detail.

As the brush just modifies displacement values instead of coordinates,
the total displacement of the affected area doesn't change. This means
that this smearing effect can be used multiple times over the same area
without generating any artifacts in the topology.

When the brush is used with the pinch or expand smear modes,
displacement differences are pushed into the same area, creating hard
surface effects without pinching the topology.

As any other brush that relies on the limit surface (like displacement
erasers), this will work better after using apply base.

Reviewed By: sergey, JulienKaspar, dbystedt

Differential Revision: https://developer.blender.org/D9659