wayland: Update focus window during DnD motion

But don't emit crossing events, since the DnD grab is in effect.

https://bugzilla.gnome.org/show_bug.cgi?id=728030

src/wayland/meta-wayland-data-device.c

@@ -212,6 +212,8 @@ drag_grab_focus (MetaWaylandPointerGrab *grab,
   if (drag_grab->drag_focus == surface)
     return;
 
+  meta_wayland_pointer_set_focus (&seat->pointer, surface, FALSE);
+
   if (drag_grab->drag_focus_data_device)
     {
       wl_data_device_send_leave (drag_grab->drag_focus_data_device);