clutter: Handle grabs from CLUTTER_TABLET_DEVICE devices

Those would result in warnings. Actually, these should be dealt
similarly to pointers, as they get their own standalone pointer
cursor in Wayland.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/issues/540

clutter/clutter/clutter-main.c

diff --git a/clutter/clutter/clutter-main.c b/clutter/clutter/clutter-main.c
index 46537f3..1c1d6f7 100644
--- a/clutter/clutter/clutter-main.c
+++ b/clutter/clutter/clutter-main.c
@@ -2757,6 +2757,7 @@
   switch (device->device_type)
     {
     case CLUTTER_POINTER_DEVICE:
+    case CLUTTER_TABLET_DEVICE:
       grab_actor = &(device->pointer_grab_actor);
       break;
 
@@ -2802,6 +2803,7 @@
   switch (device->device_type)
     {
     case CLUTTER_POINTER_DEVICE:
+    case CLUTTER_TABLET_DEVICE:
       grab_actor = &(device->pointer_grab_actor);
       break;