device/xi2: Implement get_core_device()

We ask XI2 to get the client pointer for CLUTTER_POINTER_DEVICE, and
we use the attached keyboard device for CLUTTER_KEYBOARD_DEVICE. For
everything else, we return NULL.

clutter/x11/clutter-device-manager-xi2.c

@@ -896,6 +896,31 @@ static ClutterInputDevice *
 clutter_device_manager_xi2_get_core_device (ClutterDeviceManager   *manager,
                                             ClutterInputDeviceType  device_type)
 {
+  ClutterDeviceManagerXI2 *manager_xi2 = CLUTTER_DEVICE_MANAGER_XI2 (manager);
+  ClutterBackendX11 *backend_x11;
+  ClutterInputDevice *device;
+  int device_id;
+
+  backend_x11 =
+    CLUTTER_BACKEND_X11 (_clutter_device_manager_get_backend (manager));
+
+  XIGetClientPointer (backend_x11->xdpy, None, &device_id);
+
+  device = g_hash_table_lookup (manager_xi2->devices_by_id,
+                                GINT_TO_POINTER (device_id));
+
+  switch (device_type)
+    {
+    case CLUTTER_POINTER_DEVICE:
+      return device;
+
+    case CLUTTER_KEYBOARD_DEVICE:
+      return clutter_input_device_get_associated_device (device);
+
+    default:
+      break;
+    }
+
   return NULL;
 }