Rendering does not use correct nondefault theme for anonymous

Currently the theme and theme token are not sent as part of the request, due to the removal of ajax_page_state. However, due to the checks carried out in menu_get_custom_theme() we require these variables for the correct theme to be used for rendering.

I saw this problem manifest when using an administrative AJAX view. The HTML returned was being rendered using a template from the front-end theme rather than the admin theme, because theme detection couldn't be performed from the request.

The fix here is to at least pass the theme and theme_token along with the request, and alter the theme detection to allow GET to be used instead of POST (see ajax_base_page_theme()). This will make the caching less effective given multiple sessions and distinct theme tokens, but will make the behaviour correct (which I would argue beats better caching!).