Update to latest versions of all dependencies. Among other things, this
gets us on piet 0.2, though almost all of the changes were around text,
which is not yet implemented.
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.
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).
