fix:RTTI head loss (#1675)

Co-authored-by: hujin <hj@mapgis.com>

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -1832,7 +1832,7 @@ internal static bool {Helpers.TryGetNativeToManagedMappingIdentifier}(IntPtr nat
             string suffix = (destructorOnly ? "_dtor" : string.Empty) +
                 (tableIndex == 0 ? string.Empty : tableIndex.ToString(CultureInfo.InvariantCulture));
 
-            WriteLine($"{table}[{tableIndex}] = CppSharp.Runtime.VTables.CloneTable(SafeHandles, instance, {vptrOffset}, {entries.Count});");
+            WriteLine($"{table}[{tableIndex}] = CppSharp.Runtime.VTables.CloneTable(SafeHandles, instance, {vptrOffset}, {entries.Count}, {offsetRTTI});");
 
             // fill the newly allocated v-table
             for (var i = 0; i < entries.Count; i++)

src/Runtime/VTables.cs

@@ -46,15 +46,15 @@ namespace CppSharp.Runtime
             }
         }
 
-        public unsafe static IntPtr* CloneTable(List<SafeUnmanagedMemoryHandle> cache, IntPtr instance, int offset, int size)
+        public unsafe static IntPtr* CloneTable(List<SafeUnmanagedMemoryHandle> cache, IntPtr instance, int offset, int size, int offsetRTTI)
         {
-            var sizeInBytes = size * sizeof(IntPtr);
-            var src = ((*(IntPtr*)instance) + offset).ToPointer();
+            var sizeInBytes = (size + offsetRTTI) * sizeof(IntPtr);
+            var src = (((*(IntPtr*)instance) + offset) - offsetRTTI * sizeof(IntPtr)).ToPointer();
             var entries = (IntPtr*)Marshal.AllocHGlobal(sizeInBytes);
 
             Buffer.MemoryCopy(src, entries, sizeInBytes, sizeInBytes);
             cache.Add(new SafeUnmanagedMemoryHandle((IntPtr)entries, true));
-            return entries;
+            return entries + offsetRTTI;
         }
     }
 }