win32: Ignore client requested window move/size during SIZEMOVE

This will just be fighting the user like in e.g.
https://bugzilla.gnome.org/show_bug.cgi?id=64428

gdk/win32/gdkprivate-win32.h

@@ -343,6 +343,8 @@ extern gint		 _gdk_max_colors;
 /* TRUE while a modal sizing, moving, or dnd operation is in progress */
 extern gboolean		_modal_operation_in_progress;
 
+extern HWND		_modal_move_resize_window;
+
 /* TRUE when we are emptying the clipboard ourselves */
 extern gboolean		_ignore_destroy_clipboard;