Blender Git Loki

Paint: Smoother curve preset

This implements a 5th-order equation smoothstep, which produces a flat
surface at the brush center. Some users find that our current grab brush
is too sharp, so now we have both options.
This also improves the behavior of the new clay brushes.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6265