After this commit it should be possible to share the same deformation
formulas that are used in the Elastic Deform brush with other areas of
Blender such as Grease Pencil or proportional editing.
This also removes a lot of code from sculpt.c that is not direclty
related to sculpting.
Reviewed By: jbakker
Differential Revision: https://developer.blender.org/D6281
