Enable two-sided support for the reference area light implementation

Assets/ScriptableRenderLoop/HDRenderLoop/Shaders/Material/Lit/Lit.hlsl

         float sqrDist = dot(unL, unL);
         float3 L = normalize(unL);
 
+        // Cosine of the angle between the light direction and the normal of the light's surface.
+        float cosLNs = dot(-L, Ns);
+        cosLNs = lightData.twoSided ? abs(cosLNs) : saturate(cosLNs);
+
-        float illuminance = saturate(dot(Ns, -L)) * saturate(dot(bsdfData.normalWS, L)) / (sqrDist * lightPdf);
+        float illuminance = cosLNs * saturate(dot(bsdfData.normalWS, L)) / (sqrDist * lightPdf);
 
         float3 localDiffuseLighting = float3(0.0, 0.0, 0.0);
         float3 localSpecularLighting = float3(0.0, 0.0, 0.0);