Add a hack to reduce flickering due to undersampling

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

 void ImportanceSamplePunctualLight(float rndVal, float3 lightPosition,
                                    float3 rayOrigin, float3 rayDirection,
                                    float tMin, float tMax,
-                                   out float dist, out float rSq, out float rcpPdf)
+                                   out float dist, out float rSq, out float rcpPdf,
+                                   float minDistSq = FLT_EPS)
-    float  rayToLightDistSq    = max(originToLightDistSq - originToLightProj * originToLightProj, FLT_EPS);
+    float  rayToLightDistSq    = max(originToLightDistSq - originToLightProj * originToLightProj, minDistSq);
 
     float a    = tMin - originToLightProj;
     float b    = tMax - originToLightProj;

ScriptableRenderPipeline/HDRenderPipeline/Lighting/Volumetrics/Resources/VolumetricLighting.compute

 
                     if (sampleLight)
                     {
+                        // We are unable to adequately sample features larger
+                        // than the half of the length of the integration interval
+                        // divided by the number of temporal samples (7).
+                        // Therefore, we apply this hack to reduce flickering.
+                        float hackMinDistSq = Sq(dt * (0.5 / 7));
+
-                                                      tEntr, tExit, t, distSq, rcpPdf);
+                                                      tEntr, tExit, t, distSq, rcpPdf,
+                                                      hackMinDistSq);
 
                         posInput.positionWS = GetPointAtDistance(ray, t);