All the real work was already done; this just needed connecting.

Bug: b/118386749
Test: dEQP-VK.pipeline.multisample.alpha_to_coverage.*
Change-Id: Ib1e2b7a64a76d8d7df9ed5c93e4cc738efdd83b3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29648Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>