x11: Handle all XI2 crossing mode values in switch

This fixes potential assertions if a GTK+ app gets to receive
a XINotifyPassiveGrab/Ungrab pointer crossing event, currently
triggerable by XIGrabEnter passive grabs.

http://bugzilla.gnome.org/show_bug.cgi?id=719762

gdk/x11/gdkdevicemanager-xi2.c

@@ -780,12 +780,16 @@ translate_crossing_mode (gint mode)
 {
   switch (mode)
     {
-    case NotifyNormal:
+    case XINotifyNormal:
       return GDK_CROSSING_NORMAL;
-    case NotifyGrab:
+    case XINotifyGrab:
+    case XINotifyPassiveGrab:
       return GDK_CROSSING_GRAB;
-    case NotifyUngrab:
+    case XINotifyUngrab:
+    case XINotifyPassiveUngrab:
       return GDK_CROSSING_UNGRAB;
+    case XINotifyWhileGrabbed:
+      /* Fall through, unexpected in pointer crossing events */
     default:
       g_assert_not_reached ();
     }