- Descriptor set pool allocation did not take the layout pointer into
  account.
- Allocating descriptor sets from the pool was not tracking the first
  node.
- The pointer to a second allocation node was offset from null instead
  of the pool base address.
- Added assert that the descriptor update type matches the layout.
- Refactoring to avoid duplicate casting of handles.

Bug b/123244275

Tests: dEQP-VK.api.object_management.*
Change-Id: Idf7aeb8d2597b30038ba1e9e371d99f09639f13c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28230
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>