DRW : Add new view_update mechanism.

This makes updates for the viewport cleaner and also add the possibility to add a new callback called when the scene is updated.

source/blender/draw/modes/edit_mesh_mode.c

@@ -593,5 +593,6 @@ DrawEngineType draw_engine_edit_mesh_type = {
 	&EDIT_MESH_cache_populate,
 	NULL,
 	NULL,
-	&EDIT_MESH_draw_scene
+	&EDIT_MESH_draw_scene,
+	NULL,
 };