vulkan: implement proper descriptor set handling

vulkan: implement proper descriptor set handling

The major functionality gain this provides is proper reference counting
for a descriptor set.  Overall this allows us to create descriptor sets
when they are needed (or reused from a cache) without violating any of
vulkan's object synchronisation requirements.

As there are a fixed number of sets available in a pool, the number of
descriptors in elements is currently hardcoded to 32.  This can be extended
in a future change to create pools on the fly if that limit is ever overrun.