Adapt to painting and picking API change

While still leaving them unused, pass around ClutterPaintContext and
ClutterPickContext when painting and picking.

The reason for splitting this change up in two is to make it possible to
bisect easier in between the API change and the change to using the
framebuffer passed around with the temporary contexts.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/827

js/ui/userWidget.js

@@ -142,8 +142,8 @@ class UserWidgetLabel extends St.Widget {
         this._currentLabel.allocate(childBox, flags);
     }
 
-    vfunc_paint() {
-        this._currentLabel.paint();
+    vfunc_paint(paintContext) {
+        this._currentLabel.paint(paintContext);
     }
 
     _updateUser() {