Fix incorrect Face Sets when using mask extract

Mask extract modifies the topology when adding the boundary loop, so
previous face sets may not be correct in the new mesh. Remove the face
sets datalayer and let sculpt mode rebuild it when entering sculpt mode
in the new object.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7129

source/blender/editors/mesh/editmesh_mask_extract.c

@@ -212,6 +212,11 @@ static int paint_mask_extract_exec(bContext *C, wmOperator *op)
   Object *new_ob = ED_object_add_type(C, OB_MESH, NULL, ob->loc, ob->rot, false, local_view_bits);
   BKE_mesh_nomain_to_mesh(new_mesh, new_ob->data, new_ob, &CD_MASK_EVERYTHING, true);
 
+  /* Remove the Face Sets as they need to be recreated when entering Sculpt Mode in the new object.
+   * TODO(pablodobarro): In the future we can try to preserve them from the original mesh. */
+  Mesh *new_ob_mesh = new_ob->data;
+  CustomData_free_layers(&new_ob_mesh->pdata, CD_SCULPT_FACE_SETS, new_ob_mesh->totpoly);
+
   if (RNA_boolean_get(op->ptr, "apply_shrinkwrap")) {
     BKE_shrinkwrap_mesh_nearest_surface_deform(C, new_ob, ob);
   }