This just adds the first step of polyline stroking, which is adding it
to the scene. Also just a bit of cleaning up of dimensions into one
header file.
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).
