Describe the pull request

In the Vulkan backend, descriptor sets are allocated from fixed-size descriptor pools. If descriptor sets are allocated from descriptor pools, they eventually run out of memory. This causes new pools to be allocated. To prevent unnecessary allocations, the pool size could be configured by clients. The default pool size is pretty conservatively assumed at 1024 descriptor sets per pool. Most descriptor sets are, however, only allocated sparingly, so a lot of space is wasted.

With this PR, pool sizes are determined by the amount of descriptor sets to be allocated. If a descriptor set layout needs to allocate large quantities of descriptor sets, it is now up to the client to allocate them in larger chunks. The favoured way would be to allocate a large set of descriptor sets (using allocateMultiple) and discard them immediately. This will cause the descriptor sets to be cached until they are requested later on, when they will be handed out again.

However, most descriptor sets are only created infrequently (ideally once) in small quantities, as this is the preferred way of using them. In this case, the new approach will no longer cause excessive pool sizes.

The poolSize parameters have been removed from the descriptor set layout and its builder interface.

Related issues

• Closes Make descriptor pool sizes dynamic. #108.