window: Update drag hotspot as new touches approach

Any number of touches >= 3 starts a drag operation, but the
window jumped around as new touches joined. So recalculate
the touch drag area/hotspot and tiling mode as expected
as soon as touches enter/leave the drag op.

src/core/display-private.h

@@ -161,6 +161,7 @@ struct _MetaTouchInfo
   gdouble initial_root_y;
 
   guint notified : 1;
+  guint use_for_hotspot : 1;
 };
 
 struct _MetaDisplay