toNode
Returns the render node that was constructed by @snapshot.
Note that this function may return null if nothing has been added to the snapshot or if its content does not produce pixels to be rendered.
After calling this function, it is no longer possible to add more nodes to @snapshot. The only function that should be called after this is method@GObject.Object.unref.
Return
the constructed GskRenderNode
or null if there are no nodes to render.