clutter/text: Use new pango API to compare attribute lists
Use the new API provided by Pango to compare two PangoAttrLists, this means we can also detect equal lists that were newly created and avoid the expensive relayout of the ClutterText a lot more often.
Depends on pango!172 (merged)
Fixes gnome-shell#1411 (closed)
This supersedes !983 (merged) and avoids the list-copying done in that MR.
Edited by Robert Mader