layers:Add validation to DestroyDescriptorPool

layers:Add validation to DestroyDescriptorPool

Add validation flag for DestroyDescriptorPool and update it with
validation code using the Pre/Post pattern.
Flag error if descriptor pool is in use and invalidate any cmd buffers
that it was bound to. Remove pool state from map when it's destroyed.
When the pool is destroyed make sure all of its descriptor sets are
also freed.