Chipping away at the dx12 backend. This should more or less do the
signalling to the CPU that the command buffer is done (ie wire up the
fence). It also creates buffer objects.
The hub does a little better lifetime tracking of resources (so
Rust-side references can be dropped), and in the future will be used for
dynamic selection of backend.
The migration is still a bit half-baked, as there are a bunch of
Vulkan-specific types in the signatures, but it shouldn't be too much
work to sort that out. Perhaps it can wait until there is a second
backend though.
The main motivation for this is to create image objects with lifetime
tracking, one of the things required for #38.
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.
