Improve the remapping of the FGD texture

ScriptableRenderPipeline/Core/ShaderLibrary/ImageBasedLighting.hlsl

         }
     }
 
-    // Remap from [0, 1.5] to [0, 1] range.
-    acc.z /= 1.5;
+    acc /= sampleCount;
-    return acc / sampleCount;
+    // Remap from the [0.5, 1.5] to the [0, 1] range.
+    acc.z -= 0.5;
+
+    return acc;
 }
 
 uint GetIBLRuntimeFilterSampleCount(uint mipLevel)

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

 #ifdef LIT_DIFFUSE_LAMBERT_BRDF
     diffuseFGD = 1.0;
 #else
-    // Remap from [0, 1] to [0, 1.5] range.
-    diffuseFGD = 1.5 * preFGD.z;
+    // Remap from the [0, 1] to the [0.5, 1.5] range.
+    diffuseFGD = preFGD.z + 0.5;
 #endif
 
     reflectivity = preFGD.y;
     // envDiffuseLighting is used for refraction in this Lit material. Use the weight to balance between transmission and reflection
     diffuseLighting = lerp(directDiffuseLighting + bakeDiffuseLighting, accLighting.envDiffuseLighting, accLighting.envDiffuseLightingWeight);
     specularLighting = accLighting.dirSpecularLighting + accLighting.punctualSpecularLighting + accLighting.areaSpecularLighting + accLighting.envSpecularLighting;
+    // Rescale the GGX to account for the multiple scattering.
     specularLighting *= 1 + bsdfData.fresnel0 * preLightData.energyCompensation;
 }