Fix visual artifacts with partially hidden meshes and mask extract

The previous behaivour didn't make sense as sculpt mode was still active
when switching to the new object, so it was rendering inconrrectly.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7130

source/blender/editors/mesh/editmesh_mask_extract.c

@@ -80,6 +80,9 @@ static int paint_mask_extract_exec(bContext *C, wmOperator *op)
   View3D *v3d = CTX_wm_view3d(C);
   Scene *scene = CTX_data_scene(C);
 
+  Depsgraph *depsgraph = CTX_data_depsgraph_on_load(C);
+  ED_object_sculptmode_exit(C, depsgraph);
+
   BKE_sculpt_mask_layers_ensure(ob, NULL);
 
   Mesh *mesh = ob->data;