layers:Add pool member to DescriptorSet class

layers:Add pool member to DescriptorSet class

When creating a DescriptorSet instance, record the pool that the set is
allocated from in a private pool_ variable.
Then, when binding a set to a cmd buffer, also create a binding between
the command buffer and the pool.
This allows us to catch the in-use case when sets are implicitly freed
by calling vkDestroyDescriptorPool().