`GskRenderNode` is the basic block in a scene graph to be rendered using [class@Gsk.
Renderer].
Each node has a parent, except the top-level node; each node may have children nodes.
Each node has an associated drawing surface, which has the size of the rectangle set when creating it.
Render nodes are meant to be transient; once they have been associated to a [[email protected]] it's safe to release any reference you
have on them. All [[email protected]]s are immutable, you can only specify their properties during construction.