Rework __OriginalVTables to be unique per instance.

We need to take into account that the original vtables might differ per object instance.

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -1312,7 +1312,7 @@ namespace CppSharp.Generators.CSharp
 
             const string dictionary = "System.Collections.Generic.Dictionary";
 
-            WriteLine("private static void*[] __OriginalVTables;");
+            WriteLine("private void*[] __OriginalVTables;");
             WriteLine("private static void*[] __ManagedVTables;");
             WriteLine("private static void*[] _Thunks;");
             WriteLine("private static {0}<IntPtr, WeakReference> _References;", dictionary);