[Experimental] Render Graph v1

[DasLixou]

Draft PR? yeah, this is not directly for merge. Instead, this is a research of how a Render Graph for vello could look like. My thoughts are below, please share yours.

Other than most of the other graphical application who drive a render graph, vello needs to know the size (for a buffer) or width,heigth (for an image) for a resource, while it creates them dynamically in the render nodes/stages. So a simple "just create a static buffer of that size and give it to this node" doesn't quite work. What I have done is somewhat quite overengineered/overkill I think, so I would love to hear simpler ideas from you. Basically, every node has an output struct and when linking the nodes, you can already specify how the outcome of a dependency will be used for the node. Then with some type erasure magic I store all the outputs of the nodes and when all dependencies are resolved, we cast them back for the node to get its inputs.

I haven't removed the old code in render.rs, but it isn't used anymore. Also, I just now see how many resources we create, I wonder how long we are able to run before we run out of the u64 (probably loooong, but still feels itchy).

Also, I implemented some cool helpers for recording such as that now, dispatch doesn't get an array, but rather a tuple where we can now pass different impl Into<ResourceProxy> structs like ImageProxy, BufferProxy, ResourceProxy at once without having to call .into() on the different ones.

All special stuff like reusing resources isn't implemented yet (where I also wonder how we would do that exactly... because even with "the same buffer" (being the same usage) could just be bigger on the next iteration.. hmm)

So yeah just a basic prototype, which at least runs, to get one example of how it could look and gather feedback. :>