The hub does a little better lifetime tracking of resources (so
Rust-side references can be dropped), and in the future will be used for
dynamic selection of backend.
The migration is still a bit half-baked, as there are a bunch of
Vulkan-specific types in the signatures, but it shouldn't be too much
work to sort that out. Perhaps it can wait until there is a second
backend though.
The main motivation for this is to create image objects with lifetime
tracking, one of the things required for #38.
This patch adds a module that contains both scene and ptcl types (very
lightly adapted from piet-metal), as well as infrastructure for encoding
Rust-side.
WIP, it's not wired up in either the shader or on the Rust side.
Populates the piet-gpu subdir, with an extremely simple renderer. The
main program saves the image to a PNG.
Contains a few fixes (I was confused about the need for multiple
bindings, as opposed to multiple descriptors within a binding).
