add plywood texture as a constant, as we need it in a couple of places

and it's super cryptic to have random UUIDs hanging out.

OpenSim/Framework/Constants.cs

@@ -31,6 +31,7 @@ namespace OpenSim.Framework
     {
         public const uint RegionSize = 256;
         public const byte TerrainPatchSize = 16;
+        public const string DefaultTexture = "89556747-24cb-43ed-920b-47caed15465f";
 
         public enum EstateAccessCodex : uint
         {