Use the isotropic phase function as an optimization

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

 #define SHADERPASS    SHADERPASS_VOLUMETRIC_LIGHTING
 #define GROUP_SIZE_1D 16
 #define GROUP_SIZE_2D (GROUP_SIZE_1D * GROUP_SIZE_1D)
+// #define USE_HENYEY_GREENSTEIN_PHASE_FUNCTION
 
 //--------------------------------------------------------------------------------------------------
 // Included headers
 {
     float3 scattering = _GlobalFog_Scattering;
     float  extinction = _GlobalFog_Extinction;
+#ifdef USE_HENYEY_GREENSTEIN_PHASE_FUNCTION
+#endif
 
     LightLoopContext context;
     // ZERO_INITIALIZE(LightLoopContext, context);
                 float  intensity = 1;
                 float3 color     = lightData.color;
 
+            #ifdef USE_HENYEY_GREENSTEIN_PHASE_FUNCTION
+            #endif
 
                 [branch] if (lightData.shadowIndex >= 0)
                 {
                 float  intensity     = GetPunctualShapeAttenuation(lightData, L, distSq);
                 float3 color         = lightData.color;
 
+            #ifdef USE_HENYEY_GREENSTEIN_PHASE_FUNCTION
+            #endif
                 intensity *= Transmittance(OpticalDepthHomogeneous(extinction, dist));
 
                 [branch] if (lightData.shadowIndex >= 0)
 
         radiance += sampleRadiance * (Transmittance(OpticalDepthHomogeneous(extinction, t)) * dt);
     }
-
+#ifdef USE_HENYEY_GREENSTEIN_PHASE_FUNCTION
+#else
+    float3 phaseConstant = scattering * IsotropicPhaseFunction();
+#endif
     return radiance * phaseConstant;
 }