Sculpt: Grab silhouette option

This adds a property to the grab that masks vertices based on its
original normal and grab delta. When used on thin meshes, it allows to
grab the silhouette from one side of the object without affecting the
shape of the other side.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9205

release/scripts/startup/bl_ui/properties_paint_common.py

@@ -698,6 +698,7 @@ def brush_settings(layout, context, brush, popover=False):
 
         elif sculpt_tool == 'GRAB':
             layout.prop(brush, "use_grab_active_vertex")
+            layout.prop(brush, "use_grab_silhouette")
 
         elif sculpt_tool == 'PAINT':
             row = layout.row(align=True)