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>