st: Switch to render nodes

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
11 jobs for !1538 with gbsneto/paint-node in 3 minutes and 7 seconds
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