gtkwindow.c was not enough modified when the window icon was changed from a
GdkPixbuf to a cairo_surface_t.

gtk_window_set_property(), case PROP_ICON:
  g_value_get_object() shall be g_value_get_boxed().

gtk_window_get_property(), case PROP_ICON:
  g_value_set_object() shall be g_value_set_boxed().

icon_from_list():
  Callers expect this function to add a reference to the returned icon,
  but this is missing in one place. Change
    best = (cairo_surface_t *)list->data;
  to
    best = cairo_surface_reference ((cairo_surface_t *)list->data);

gtk_window_set_icon():
    g_return_if_fail (icon == NULL);
  makes it impossible to set an icon with this function. Either remove that
  statement or replace it with
    g_return_if_fail (icon == NULL ||
      cairo_surface_get_type (icon) == CAIRO_SURFACE_TYPE_IMAGE);
  in analogy with what gtk_window_set_default_icon() does.