Sculpt: Elastic Deform Brush

This patch implements the paper "Regularized Kelvinlets: Sculpting Brushes based on Fundamental Solutions of Elasticity" https://graphics.pixar.com/library/Kelvinlets/paper.pdf

It includes grab, biscale grab, triscale grab, scale and twist.
All deformation modes are accessible under the same tool. This helps to keep the code organized and it should not make any difference to the user when a better brush management system is implemented.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5634

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -369,6 +369,13 @@ class VIEW3D_PT_tools_brush(Panel, View3DPaintPanel):
             row = col.row()
             row.prop(brush, "normal_radius_factor", slider=True)
 
+            if brush.sculpt_tool == 'ELASTIC_DEFORM':
+                col.separator()
+                row = col.row()
+                row.prop(brush, "elastic_deform_type")
+                row = col.row()
+                row.prop(brush, "elastic_deform_compressibility", slider=True)
+
             # topology_rake_factor
             if (
                     capabilities.has_topology_rake and