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
A generic, introspectable Shader effect is not only more flexible
than a shader actor, it will also make it much easier to turn
Lightbox into an actor subclass and replace Tweener with Clutter's
own animation support.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/651
