The idea of having a paint node tree is that we don't really need
to retrieve the framebuffer from ClutterPaintContext. For example,
ClutterLayerNode draws into an offscreen framebuffer; if any child
of a layer node needs to retrieve a framebuffer to draw, the layer
node's offscreen framebuffer should be used.
However, clutter_paint_node_get_framebuffer() goes straight to the
root node of the tree, skipping any potential paint nodes with a
custom framebuffer.
Modify clutter_paint_node_get_framebuffer() to walk up the paint
node tree until a node with a custom framebuffer appears. In many
cases, this will end up either in dummy or layer node's custom
framebuffer implementations.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1340>
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