Introduce a new paint_node vfunc that, if implemented, allows
the effect to add nodes to a transient paint node that is
immediately painted. This is a transitional step until we
have fully delegated paint node rendering.
The most basic implementation of a ClutterEffect.paint_node
vfunc, and also the default implementation, is with an actor
node, as follows:
```
static void
foo_bar_paint_node (ClutterEffect *effect,
ClutterPaintNode *node,
ClutterPaintContext *paint_context,
ClutterEffectPaintFlags flags)
{
g_autoptr (ClutterPaintNode) actor_node = NULL;
actor_node = clutter_actor_node_new (effect->actor);
clutter_paint_node_add_child (node, actor_node);
}
```
This example gives the exact same behavior of simply calling
clutter_actor_continue_paint(). In the future, the paint node
itself will be a parameter of clutter_actor_continue_paint()
and we'll be able to simplify it event more.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
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