Blender Git Loki

Sculpt: Elastic deform type for Snake Hook

This adds deformation types to snake hook and the elastic deformation
type. This mode deforms the mesh using a kelvinlet instead of applying
the displacement directly inside the brush radius, which is great for
stylized shapes sketching.

Changes in rake rotation when using elastic are too strong when set
to 1, so I'll add a nicer way to support rake rotations with smoother
transitions in the future.

Reviewed By: sergey, JulienKaspar

Differential Revision: https://developer.blender.org/D9560