Fix numerical integration issues

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

 
         BSDF(V, L, positionWS, preLightData, bsdfData, lightDiff, lightSpec);
 
+        // The value of the specular BSDF could be infinite.
+        // Summing up infinities leads to NaNs.
+        lightSpec = min(lightSpec, FLT_MAX);
+
         diffuseLighting  += lightDiff * (sinLT / dist2 * NdotL);
         specularLighting += lightSpec * (sinLT / dist2 * NdotL);
     }

Assets/ScriptableRenderLoop/ShaderLibrary/Common.hlsl

 #define HALF_PI     1.57079632679
 #define INV_HALF_PI 0.636619772367
 
-#define FLT_EPSILON     1.192092896e-07f // smallest such that 1.0 + FLT_EPSILON != 1.0
+#define FLT_EPSILON 1.192092896e-07 // Smallest positive number, such that 1.0 + FLT_EPSILON != 1.0
+#define FLT_MAX     3.402823466e+38 // Maximum representable floating-point number
 
 #define MERGE_NAME(X, Y) X##Y