This was motivated by experiments with the Vulkan memory model. To use
that, we actually need to explicitly enable the relevant feature on
device creation time. That's a lot easier to do now that push_next works
on the structs in that chain. This PR doesn't do that though, it only
upgrades the dependency and cleans up deprecations.
Adds a new "mux" module which can have multiple backends. As of this
commit, it's not wired up at all, but the functionality should be
reasonably complete.
Minor tweaks to the backend trait to accommodate this, mostly changing
Fence and Semaphore to references so they don't need to be Copy.
Part of the work toward #95
Don't run extensions unless they're available. This includes querying
for descriptor indexing, and running one of two versions of kernel4
depending on whether it's enabled.
Part of the support needed for #78
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.
