This enables deriving the state structure from Memset<> so we can ensure
it is fully zero-initialized and becomes comparable, while vk::Sampler
holds the identifier for the state.

Bug: b/151235334
Change-Id: I23a09eda9b50409c761bdd6ed10911ec159dc3ba
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42768
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>