Improve the formulation of the dominant specular dir

Assets/ScriptableRenderLoop/ShaderLibrary/ImageBasedLighting.hlsl

 // Ref: "Moving Frostbite to PBR", p. 69.
 float3 GetSpecularDominantDir(float3 N, float3 R, float roughness, float NdotV)
 {
-    float a     = 1.0 - roughness;
-    float s     = sqrt(1.0 - roughness);
-    float sinNV = sqrt(1.0 - NdotV * NdotV);
+    float a = 1.0 - roughness;
+    float s = sqrt(a);
-    float lerpFactor = a * (s + roughness);
+    float lerpFactor = (s + roughness) * a;
-    float lerpFactor = s * saturate(lerp(s + s, a * a, sinNV));
+    float lerpFactor = (s + roughness) * saturate(lerp(a * a, a * a + a, NdotV * NdotV));
 #endif
 
     // The result is not normalized as we fetch in a cubemap