Implement faster (achromatic) bilateral weighting

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

             #endif
                 float2 scaledDirection = distScale * stepSize * unitDirection;
 
-                // Load (1 / (2 * WeightedVariance)) for bilateral weighting.
+                // Load (1 / (2 * Variance)) for bilateral weighting.
+            #ifdef RBG_BILATERAL_WEIGHTS
-
+            #else
+                float  halfRcpVariance = _FilterKernels[profileID][N_SAMPLES].a;
+            #endif
                 // Take the first (central) sample.
                 float2 samplePosition   = posInput.unPositionSS;
                 float3 sampleWeight     = _FilterKernels[profileID][0].rgb;
                     rawDepth         = LOAD_TEXTURE2D(_CameraDepthTexture, samplePosition).r;
                     sampleIrradiance = LOAD_TEXTURE2D(_IrradianceSource,   samplePosition).rgb;
 
-                    // Apply bilateral filtering.
+                    // Apply bilateral weighting.
                     // Ref #1: Skin Rendering by Pseudo–Separable Cross Bilateral Filtering.
                     // Ref #2: Separable SSS, Supplementary Materials, Section E.
                     float sampleDepth = LinearEyeDepth(rawDepth, _ZBufferParams);

Assets/ScriptableRenderLoop/HDRenderPipeline/SceneSettings/SubsurfaceScatteringParameters.cs

                 m_FilterKernel[i].z *= 1 / weightSum.z;
             }
 
-            Vector3 weightedStdDev;
+            Vector4 weightedStdDev;
+            weightedStdDev.w = Mathf.Lerp(maxStdDev1,  maxStdDev2,  m_LerpWeight);
-            // Store (1 / (2 * WeightedVariance)) per color channel.
+            // Store (1 / (2 * Variance)) per color channel.
+            m_FilterKernel[numSamples].w = 0.5f / (weightedStdDev.w * weightedStdDev.w);
         }
     }