Don't use get_core_device

clutter_device_manager_get_core_device calls XIGetClientPointer, which
requires a round-trip to the server. Since we do this on StWidget
creation, this means a full round-trip for every created StWidget.
Replace this with get_device with the ID of the VCP/VCK, since mutter
doesn't support MPX, and we know this is what the device is.

src/shell-screenshot.c

@@ -253,7 +253,7 @@ get_pointer_coords (int *x,
   ClutterPoint point;
 
   manager = clutter_device_manager_get_default ();
-  device = clutter_device_manager_get_core_device (manager, CLUTTER_POINTER_DEVICE);
+  device = clutter_device_manager_get_device (manager, META_VIRTUAL_CORE_POINTER_ID);
 
   clutter_input_device_get_coords (device, NULL, &point);
   *x = point.x;