We currently create statically sized descriptor pools, shared among
different descriptor types. Once we're unable to allocate a descriptor
set from a pool, we create a new pool. The unfortunate but predictable
consequence is that when we run out of descriptors of one type, we waste
any unallocated descriptors of the other types.
Dynamically adjusting the pool sizes could mitigate the issue, but it
seems non-trivial to handle all the edge cases, particularly in
situations where the descriptor count ratios change significantly
between frames. Instead, by storing only a single vkd3d descriptor type
in each Vulkan descriptor set we're able to create separate descriptor
pools for each vkd3d descriptor type, which also avoids the issue.
The main drawback of using separate descriptor sets for each descriptor
type is that we can no longer pack all bounded descriptor ranges into a
single descriptor set, potentially leaving fewer descriptor sets
available for unbounded ranges. That seems worth it, but we may end up
having to switch to a more complicated strategy if this ends up being a
problem on Vulkan implementations with a very limited number of
available descriptor sets.
