Updated empirical mip remaping for refraction and planar reflection

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/ImageBasedLighting.hlsl

     return PerceptualRoughnessToMipmapLevel(perceptualRoughness, UNITY_SPECCUBE_LOD_STEPS);
 }
 
-real PerceptualRoughnessToMipmapLevelFromGaussianConvolution(real perceptualRoughness, uint mipMapCount)
-{
-    return sqrt(perceptualRoughness) * mipMapCount;
-}
-
 // The *accurate* version of the non-linear remapping. It works by
 // approximating the cone of the specular lobe, and then computing the MIP map level
 // which (approximately) covers the footprint of the lobe with a single texel.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.hlsl

     preLightData.transparentTransmittance = exp(-bsdfData.absorptionCoefficient * refraction.dist);
     // Empirical remap to try to match a bit the refraction probe blurring for the fallback
     // Use IblPerceptualRoughness so we can handle approx of clear coat.
-    preLightData.transparentSSMipLevel = PerceptualRoughnessToMipmapLevelFromGaussianConvolution(preLightData.iblPerceptualRoughness, uint(_ColorPyramidScale.z));
+    preLightData.transparentSSMipLevel = pow(preLightData.iblPerceptualRoughness, 1.3) * uint(max(_ColorPyramidScale.z - 1, 0));
 #endif
 
     return preLightData;
     // Specific case for Texture2Ds, their convolution is a gaussian one and not a GGX one.
     // So we use another roughness mip mapping.
     if (IsEnvIndexTexture2D(lightData.envIndex))
-        iblMipLevel = PerceptualRoughnessToMipmapLevelFromGaussianConvolution(preLightData.iblPerceptualRoughness, uint(_ColorPyramidScale.z));
+        // Empirical remapping
+        iblMipLevel = pow(preLightData.iblPerceptualRoughness, 0.8) * uint(max(_ColorPyramidScale.z - 1, 0));
 
     float4 preLD = SampleEnv(lightLoopContext, lightData.envIndex, R, iblMipLevel);
     weight *= preLD.a; // Used by planar reflection to discard pixel