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/gdkglobals-win32.c

@@ -79,6 +79,7 @@ gint		  _gdk_input_ignore_wintab = FALSE;
 gint		  _gdk_max_colors = 0;
 
 gboolean	  _modal_operation_in_progress = FALSE;
+HWND              _modal_move_resize_window = NULL;
 gboolean	  _ignore_destroy_clipboard = FALSE;
 
 HGLOBAL           _delayed_rendering_data = NULL;