Added missing case for a single sample with alphaToCoverage enabled Change-Id: Iad9234fdd30d64c60ae189c123564a0423ab12df Tests: dEQP-VK.rasterization.frag_side_effects.color_at_* Bug: b/168046415 Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48209 Presubmit-Ready: Alexis Hétu <sugoi@google.com> Kokoro-Result: kokoro <noreply+kokoro@google.com> Tested-by: Alexis Hétu <sugoi@google.com> Reviewed-by: Nicolas Capens <nicolascapens@google.com>
