Optimize ImportanceSamplePunctualLight() - more work is needed

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

 }
 
 // Implements equiangular light sampling.
-// Returns the distance from 0 and the reciprocal of the PDF.
+// Returns the distance from the origin of the ray, the squared (radial) distance from the light,
+// and the reciprocal of the PDF.
-                                   out float dist, out float rcpPdf)
+                                   out float dist, out float rSq, out float rcpPdf)
-    float  rayToLightDist      = sqrt(rayToLightDistSq);
-    float a = tMin - originToLightProj;
-    float b = tMax - originToLightProj;
-    float d = rayToLightDist;
+    float a    = tMin - originToLightProj;
+    float b    = tMax - originToLightProj;
+    float dSq  = rayToLightDistSq;
+    float d    = sqrt(dSq);
+    float dInv = rsqrt(dSq);
-    float theta0 = atan(a / d);
-    float theta1 = atan(b / d);
+    float theta0 = FastATan(a * dInv);
+    float theta1 = FastATan(b * dInv);
+    float gamma  = theta1 - theta0;
-    rcpPdf = (theta1 - theta0) * (d + t * tan(theta));
+    rSq    = dSq + t * t;
+    rcpPdf = gamma * rSq * dInv;
 }
 
 // Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf

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

                     // TODO...
                     if (lightType != GPULIGHTTYPE_POINT) continue;
 
-                    float t, rcpPdf;
+                    float t, distSq, rcpPdf;
-                                                  tMin, tMax, t, rcpPdf);
+                                                  tMin, tMax, t, distSq, rcpPdf);
-                    float  distSq        = dot(lightToSample, lightToSample);
                     float  dist          = sqrt(distSq);
                     float3 L             = lightToSample * -rsqrt(distSq);
                     float  intensity     = GetPunctualShapeAttenuation(lightData, L, distSq);