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).
This version will be similar but somewhat different to what's in
piet-metal now. Hopefully a little simpler to use (no distinction
between packed and unpacked structs) and better structured.
