win32: Add gdk_win32_window_get_impl_hwnd

This is needed to fix the ms-windows theme not to create
native windows for all child windows.

gdk/gdk.symbols

@@ -354,6 +354,7 @@ gdk_win32_pixbuf_to_hicon_libgtk_only
 gdk_win32_selection_add_targets
 gdk_win32_set_modal_dialog_libgtk_only
 gdk_win32_window_is_win32
+gdk_win32_window_get_impl_hwnd
 gdk_win32_window_get_handle
 gdk_win32_window_get_type
 gdk_win32_display_get_type

gdk/win32/gdkwin32.h

@@ -80,6 +80,7 @@ G_BEGIN_DECLS
 
 /* Return true if the GdkWindow is a win32 implemented window */
 gboolean      gdk_win32_window_is_win32 (GdkWindow *window);
+HWND          gdk_win32_window_get_impl_hwnd (GdkWindow *window);
 
 /* Return the Gdk* for a particular HANDLE */
 gpointer      gdk_win32_handle_table_lookup (HWND handle);

gdk/win32/gdkwindow-win32.c

@@ -3366,6 +3366,14 @@ _gdk_win32_impl_release_dc (GdkWindowImplWin32 *impl)
     }
 }
 
+HWND
+gdk_win32_window_get_impl_hwnd (GdkWindow *window)
+{
+  if (GDK_WINDOW_IS_WIN32 (window))
+    return GDK_WINDOW_HWND (window);
+  return NULL;
+}
+
 static void
 gdk_win32_cairo_surface_destroy (void *data)
 {