Fix for multiple modifier stack calculations per frame.

Since 2.5x blender has been using CD_MASK_BAREMESH for updating objects since object_handle_update() no longer has access to G.curscreen to calculate the mask from viewports.

The problem with this is after an initial calculation, CD_MASK_MTFACE may be required on draw, so it would recalculate the modifier stack multiple times per frame.
One case which caused this is armature animated mesh with texface in a dupligroup.

Fix this by having customdata_mask member in the scene, this isn't great design but at least fixes the bug and only changes a few files.

source/blender/windowmanager/intern/wm_event_system.c

@@ -56,6 +56,7 @@
 #include "ED_fileselect.h"
 #include "ED_info.h"
 #include "ED_screen.h"
+#include "ED_view3d.h"
 #include "ED_util.h"
 
 #include "RNA_access.h"
@@ -296,6 +297,10 @@ void wm_event_do_notifiers(bContext *C)
 		/* XXX make lock in future, or separated derivedmesh users in scene */
 		if(!G.rendering)
 			/* depsgraph & animation: update tagged datablocks */
+
+			/* copied to set's in scene_update_tagged_recursive() */
+			win->screen->scene->customdata_mask= ED_viewedit_datamask(win->screen);
+
 			scene_update_tagged(CTX_data_main(C), win->screen->scene);
 	}