Fix numerical issues with the default volumetric fog mean free path

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/AtmosphericScattering/VolumetricFog.cs

     public class VolumetricFog : AtmosphericScattering
     {
         public ColorParameter        albedo       = new ColorParameter(new Color(0.5f, 0.5f, 0.5f));
-        public MinFloatParameter     meanFreePath = new MinFloatParameter(float.MaxValue, 1.0f);
+        public MinFloatParameter     meanFreePath = new MinFloatParameter(1000000.0f, 1.0f);
         public ClampedFloatParameter asymmetry    = new ClampedFloatParameter(0.0f, -1.0f, 1.0f);
 
         // Override the volume blending function.