Paint Nodes

From the commit message:

Switch to render nodes. Most of the changes follow the pattern of
replacing a cogl_framebuffer_draw_*() call by (1) creating a paint
node, and (2) calling the corresponding clutter_paint_node_add_*()

StWidget now overrides ClutterActor.paint_node(), instead of paint.
All subclasses that overrided the paint vfunc also are changed to
override paint_node.

The entry point for CSS rendering, st_widget_paint_background(), now
receives a root ClutterPaintNode, and propagates it to the rendering

Depends on mutter!1637 (merged)

Edited by Georges Basile Stavracas Neto

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