Proper singleton initialization

Ref: http://csharpindepth.com/Articles/General/Singleton.aspx

ScriptableRenderPipeline/Core/CoreRP/Volume/VolumeManager.cs

     public sealed class VolumeManager
     {
         //>>> System.Lazy<T> is broken in Unity (legacy runtime) so we'll have to do it ourselves :|
-        static volatile VolumeManager s_Instance;
-        static object s_LockObj = new UnityObject();
+        static readonly VolumeManager s_Instance = new VolumeManager();
+        public static VolumeManager instance { get { return s_Instance; } }
-        public static VolumeManager instance
-        {
-            get
-            {
-                // Double-lock checking
-                if (s_Instance == null)
-                {
-                    lock (s_LockObj) // Lock on a separate object to avoid deadlocks
-                    {
-                        if (s_Instance == null)
-                            s_Instance = new VolumeManager();
-                    }
-                }
-
-                return s_Instance;
-            }
-        }
+        // Explicit static constructor to tell the C# compiler not to mark type as beforefieldinit
+        static VolumeManager() { }
         //<<<
 
         // Internal stack