This commit implements actual rendering for the new piet-scene crate.
In particular, it adds a new EncodedSceneRef type in piet_gpu::encoder to store references to the raw stream data. This is used as a proxy for both PietGpuRenderContext and piet_scene::scene::Scene to feed data into the renderer.
Much of this is still hacky for expedience and because the intention is to be a transitional state that maintains both interfaces until we can move to the new scene API and add a wrapper for the traditional piet API.
The general structure and relationships between these crates need further discussion.
