add ofidx

src/DotRecast.Recast/PartitionType.cs

@@ -1,4 +1,5 @@
 using System.Collections.Immutable;
+using System.Linq;
 
 namespace DotRecast.Recast
 {
@@ -21,6 +22,11 @@ namespace DotRecast.Recast
             Name = name;
         }
 
+        public static PartitionType OfIdx(int idx)
+        {
+            return Values.FirstOrDefault(x => x.Idx == idx) ?? WATERSHED;
+        }
+
         public override string ToString() => Name;
     }
 }