Improve bilateral weighting and alpha blending

Assets/ScriptableRenderLoop/HDRenderPipeline/HDRenderPipeline.cs

             // Assume that the height of the projection window is 2 meters.
             float distanceToProjectionWindow = 1.0f / Mathf.Tan(0.5f * Mathf.Deg2Rad * hdCamera.camera.fieldOfView);
             m_CombineSubsurfaceScattering.SetFloat("_DistToProjWindow", distanceToProjectionWindow);
-            // Note: the user-provided bilateral scale is reversed (compared to the one in the shader).
-            m_CombineSubsurfaceScattering.SetFloat("_BilateralScale", 256.0f * (1.0f - sssParameters.bilateralScale));
+            m_CombineSubsurfaceScattering.SetFloat("_BilateralScale", 0.05f * sssParameters.bilateralScale);
             // TODO: use user-defined values for '_ProfileID' and '_FilterRadius.'
             m_CombineSubsurfaceScattering.SetVectorArray("_FilterKernel", sssParameters.profiles[0].filterKernel);
             m_CombineSubsurfaceScattering.SetFloat("_FilterRadius", 5.0f);

Assets/ScriptableRenderLoop/HDRenderPipeline/Material/Lit/Resources/CombineSubsurfaceScattering.shader

 
             ZTest  Always
             ZWrite Off
-            Blend  One [_DstBlend]
+            Blend  One [_DstBlend], Zero [_DstBlend]
 
             HLSLPROGRAM
             #pragma target 4.5
                     // Apply bilateral filtering.
                     float sDepth = LinearEyeDepth(rawDepth, _ZBufferParams);
                     float dDepth = abs(sDepth - cDepth);
-                    float dScale = _BilateralScale / (_FilterRadius * _DistToProjWindow);
-                    float t      = saturate(dScale * dDepth);
+                    float dScale = _BilateralScale * _FilterRadius * _DistToProjWindow;
+                    float t      = saturate(dDepth / dScale);
 
                     // TODO: use real-world distances for weighting.
                     filteredIrradiance += lerp(sIrradiance, cIrradiance, t) * sWeight;

Assets/ScriptableRenderLoop/HDRenderPipeline/SceneSettings/SubsurfaceScatteringParameters.cs

                     parameters.profiles[i] = SubsurfaceScatteringProfile.Default;
                 }
 
-                parameters.bilateralScale = 0.0f;
+                parameters.bilateralScale = 0.1f;
                 return parameters;
             }
         }