Clean up the shader a bit

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

                 return /* 0.25 * */ S * (expOneThird + expOneThird * expOneThird * expOneThird);
             }
 
-            // Computes F(x)/P(x), s.t. x = sqrt(r^2 + z^2).
-            float3 ComputeBilateralWeight(float3 S, float r, float z, float rcpDistScale, float rcpPdf)
+            // Computes F(x)/P(x), s.t. x = sqrt(r^2 + t^2).
+            float3 ComputeBilateralWeight(float3 S, float r, float t, float rcpDistScale, float rcpPdf)
-                float3 valX = KernelValCircle(sqrt(r * r + z * z) * rcpDistScale, S);
+                float3 valX = KernelValCircle(sqrt(r * r + t * t) * rcpDistScale, S);
-                float rcpPdfX = rcpPdf * (1 + abs(z) / r);
+                float rcpPdfX = rcpPdf * (1 + abs(t) / r);
 
                 return valX * rcpPdfX;
             }
                                                                                                 \
                 float2 position   = centerPosUnSS + vec * scaledPixPerMm;                       \
                 float  rcpPdf     = kernel[profileID][i][1];                                    \
-                float  depth      = LOAD_TEXTURE2D(_MainDepthTexture, position).r;              \
+                /* TODO: see if making this a [branch] improves performance. */                 \
-                    float  d = LinearEyeDepth(depth, _ZBufferParams);                           \
-                    float  z = millimPerUnit * d - (millimPerUnit * centerDepthVS);             \
-                    float3 w = ComputeBilateralWeight(shapeParam, r, z, rcpDistScale, rcpPdf);  \
+                    float  z = LOAD_TEXTURE2D(_MainDepthTexture, position).r;                   \
+                    float  d = LinearEyeDepth(z, _ZBufferParams);                               \
+                    float  t = millimPerUnit * d - (millimPerUnit * centerDepthVS);             \
+                    float3 w = ComputeBilateralWeight(shapeParam, r, t, rcpDistScale, rcpPdf);  \
                                                                                                 \
                     totalIrradiance += w * irradiance;                                          \
                     totalWeight     += w;                                                       \