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).
Note: also make dispatch size programmable.
Add a layout pass to the struct derive logic.
Rework Collatz example to use new traits.
This brings in a bunch of code from vk-toy but doesn't yet do anything.