The mask output from the shader is ANDed with the coverage. Slight
wrinkle here in that we have a 4 bit mask of samples in each lane, but we want to
transpose to a 4 bit mask of lane coverage per sample.

Test: dEQP-VK.renderpass.*.multisample.*
Bug: b/118386749
Change-Id: Id4d60bef71cd179915db424aa94a9d9728fb42cd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30552Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>