Allow alphaToCoverage with a single sample

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>
diff --git a/src/Device/Renderer.cpp b/src/Device/Renderer.cpp
index 9925f6e..d7b3b3f 100644
--- a/src/Device/Renderer.cpp
+++ b/src/Device/Renderer.cpp
@@ -303,6 +303,10 @@
 			data->a2c0 = float4(0.25f);
 			data->a2c1 = float4(0.75f);
 		}
+		else if(ms == 1)
+		{
+			data->a2c0 = float4(0.5f);
+		}
 		else
 			ASSERT(false);
 	}