Removes the dependency on the piet crate and replaces all uses with the scene crate. Also does some cleanup of the scene API, renaming some types and moving them all to the crate root for better ergonomics.
This removes the GPU transform stage, changes shaders to reference transforms directly from the scene, and modifies the render context to maintain a transform stack.
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.
