An ubershader archive is a bundle of filamat packages with some metadata
that conveys which glTF features each material supports.
This PR does three things:
1. Adds a new command line tool called `uberz` that consumes a list
of filamat files and metadata text files and produces a single
ubershader archive.
2. Adds a new library (also called `uberz`) that is used by `gltfio`
to read ubershader archives, and used by the above command line
tool to write ubershader archives.
3. Enhances `UbershaderLoader` so that it no longers uses a hardcoded
set of materials, and instead takes an ubershader archive.
Ubershader archives have a simple binary layout that can be memcpy'd
directly into a C struct. The metadata is specified using a text file
with key-value pairs. These two file formats have formal desriptions in
the README in `libs/uberz`.
In a subsequent PR, we will remove the `gltfio_resources` target and
change the signature of `createUbershaderLoader` so that it takes
an archive.
