Most of the code just passes the data through. The only problem
that affects all the pieces is related to memory allocation:
Vulkan expects slices, but the API gives us Iterator.
So we end up using a lot of inplace_it to get things collected on stack.
One part that has actual logic is related to framebuffers. HAL is modelled
after image-less framebuffers. If the the Vulkan implementation supports it,
we map it 1:1, and everything is great. If it doesn't expose
KHR_imageless_framebuffer, however, than we have to keep all the created
framebuffers internally in an internally-synchronized map, per B::Framebuffer.