shell-util: Fix warning in shell_util_cursor_tracker_to_clutter

If the sprite is NULL, like if a Wayland app wanted to hide the cursor,
then we need to hide the ClutterTexture on our side, as ClutterTexture
has no easy way to tell it to paint nothing.

src/shell-util.c

@@ -347,5 +347,13 @@ shell_util_cursor_tracker_to_clutter (MetaCursorTracker *tracker,
   CoglTexture *sprite;
 
   sprite = meta_cursor_tracker_get_sprite (tracker);
-  clutter_texture_set_cogl_texture (texture, sprite);
+  if (sprite)
+    {
+      clutter_actor_show (CLUTTER_ACTOR (texture));
+      clutter_texture_set_cogl_texture (texture, sprite);
+    }
+  else
+    {
+      clutter_actor_hide (CLUTTER_ACTOR (texture));
+    }
 }