Blit node & more paint node shenanigans
This MR tightens up some paint node code, and add a new
paint_node vfunc to
ClutterEffect, which will allow us to gradually port effects to the paint node API. It also adds a new
ClutterBlitNode, which was a requirement to port GNOME Shell's blur effect to the paint node API (gnome-shell!1339).
The next logical step after this MR lands is port
ClutterOffscreenEffect, and all its subclasses, to the new paint node API, as well as adding a
ClutterPaintNode *node parameter to
clutter_actor_continue_paint(). After that, a fully retained node rendering is within a stone's throw.