ClutterStage:after-paint now does not guarantee a valid
implicit framebuffer pushed to the stack. Instead, use
the new 'paint-view' signal, that is emitted at a point
in the drawing routine where a framebuffer is pushed.
In addition to that, stop using the implicit framebuffer
API and port the actor-shader-effect test to read from
the view's framebuffer directly.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/623
088117a619