Use FastACos for the area light code

Assets/ScriptableRenderLoop/HDRenderLoop/Material/Lit/Lit.hlsl

 
     // Area light specific
     // UVs for sampling the LUTs
-    // TODO: Test with fastAcos
-    float theta = acos(dot(bsdfData.normalWS, V));
+    float theta = FastACos(dot(bsdfData.normalWS, V));
     // Scale and bias for the current precomputed table - the constant use here are the one that have been use when the table in LtcData.DisneyDiffuse.cs and LtcData.GGX.cs was use
     float2 uv = 0.0078125 + 0.984375 * float2(bsdfData.perceptualRoughness, theta * INV_HALF_PI);
 

Assets/ScriptableRenderLoop/ShaderLibrary/AreaLighting.hlsl

 float IntegrateEdge(float3 v1, float3 v2)
 {
     float cosTheta = dot(v1, v2);
-    // TODO: Explain the 0.9999 <= precision is important!
-    cosTheta = Clamp(cosTheta, -0.9999, 0.9999);
-
-    // TODO: Experiment with fastAcos
-    float theta = acos(cosTheta);
-    float res = cross(v1, v2).z * theta / sin(theta);
+    // Clamp to avoid artifacts. This particular constant gives the best results.
+    cosTheta    = Clamp(cosTheta, -0.9999, 0.9999);
+    float theta = FastACos(cosTheta);
+    float res   = cross(v1, v2).z * theta / sin(theta);
 
     return res;
 }