Make "EnvLightData.envIndex" an int

Assets/ScriptableRenderLoop/HDRenderLoop/Lighting/LightDefinition.cs

         public EnvShapeType envShapeType;
 
         public Vector3 forward;
-        public float envIndex;
+        public int envIndex;
 
         public Vector3 up;
         public float blendDistance;     // blend transition outside the volume

Assets/ScriptableRenderLoop/HDRenderLoop/Lighting/LightDefinition.cs.hlsl

 	float3 positionWS;
 	int envShapeType;
 	float3 forward;
-	float envIndex;
+	int envIndex;
 	float3 up;
 	float blendDistance;
 	float3 right;
 {
 	return value.forward;
 }
-float GetEnvIndex(EnvLightData value)
+int GetEnvIndex(EnvLightData value)
 {
 	return value.envIndex;
 }