HDRenderPipeline: Minor improvement + code cleanup

- update the roughness remapping of refraction to better match
reflection probe

ScriptableRenderPipeline/Core/ShaderLibrary/CommonLighting.hlsl

 }
 
 // ior is a value between 1.0 and 2.5
-float ConvertIORToFresnel0(float ior)
+float IORToFresnel0(float ior)
 {
     return Sqr((ior - 1.0) / (ior + 1.0));
 }

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

            HenyeyGreensteinPhasePartVarying(asymmetry, LdotD);
 }
 
+// Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
+float TransmittanceColorAtDistanceToAbsorption(float3 transmittanceColor, float atDistance)
+{
+    return -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
+}
+
+
 #endif // UNITY_VOLUME_RENDERING_INCLUDED

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

     bsdfData.ior = ior;
 
     // IOR define the fresnel0 value, so update it also for consistency (and even if not physical we still need to take into account any metal mask)
-    bsdfData.fresnel0 = lerp(ConvertIORToFresnel0(ior).xxx, baseColor, metallic);
+    bsdfData.fresnel0 = lerp(IORToFresnel0(ior).xxx, baseColor, metallic);
-    // Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
-    bsdfData.absorptionCoefficient = -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
+    bsdfData.absorptionCoefficient = TransmittanceColorAtDistanceToAbsorption (transmittanceColor, atDistance);
     bsdfData.transmittanceMask = transmittanceMask;
     bsdfData.thickness = max(thickness, 0.0001);
 }
     preLightData.transmissionRefractV = refraction.rayWS;
     preLightData.transmissionPositionWS = refraction.positionWS;
     preLightData.transmissionTransmittance = exp(-bsdfData.absorptionCoefficient * refraction.distance);
-    preLightData.transmissionSSMipLevel = PerceptualRoughnessToMipmapLevel(bsdfData.perceptualRoughness, uint(_GaussianPyramidColorMipSize.z));
+    // Empirical remap to try to match a bit the refractio probe blurring for the fallback
+    preLightData.transmissionSSMipLevel = sqrt(bsdfData.perceptualRoughness) * uint(_GaussianPyramidColorMipSize.z);
 #else
     preLightData.transmissionRefractV = -V;
     preLightData.transmissionPositionWS = posInput.positionWS;