cursor-tracker/x11: Also update sprite when updating position

Without doing this, we'd use the same sprite that was last set by
mutter, most likely a leftptr cursor, and fail to update when e.g.
moving the pointer above a text entry and the displayed cursor updated
to a cursor position marker.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1391

src/backends/meta-cursor-tracker-private.h

@@ -51,6 +51,8 @@ void meta_cursor_tracker_untrack_position (MetaCursorTracker *tracker);
 
 MetaCursorSprite * meta_cursor_tracker_get_displayed_cursor (MetaCursorTracker *tracker);
 
+MetaBackend * meta_cursor_tracker_get_backend (MetaCursorTracker *tracker);
+
 void meta_cursor_tracker_notify_cursor_changed (MetaCursorTracker *tracker);
 
 #endif