To avoid re-generating sampling routines for sampler with identical
state, keep a map of sampler state to 32-bit integer identifiers.

Bug: b/151235334
Change-Id: I105151675afbf29bd29585e866b8cd976f66fb49
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42468
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>