rewrite global descriptor pool to descriptor pool allocator

It is better to have a dynamic list of medium size descriptor pools rather than one huge descriptor pool (see [DesctiptorCache](https://github.com/vkdev-team/VulkanAbstractionLayer/blob/master/VulkanAbstractionLayer/DescriptorCache.cpp#L46))

Start by reading DescriptorCache.cpp around line 46, which the issue identifies as a reference for descriptor-pool handling. Trace how the current global descriptor pool is created and used, then determine the scope and acceptance criteria for replacing it with a dynamic list of medium-sized pools; the issue does not name tests or a more specific entry point.

Written by the indexing model from the issue text.