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.
Serve two purposes:
1. refcounting of vulkan handles with associated destruction. When
combined with the trash list, the user can ensure destruction at
the correct time according to the vulkan rules.
2. avoids polluting our API with 32-bit vs 64-bit integer/pointers
differences as exposed through the vulkan API. on 32-bit, vulkan
non-dispatchable handles are 64-bit integers and on 64-bit, they
are pointers.
