Filter gateway checkout form output

Gateway checkout form fields are normally generated by the gateway early on in the WordPress load cycle. From my testing it looks like this happens too early for things like the wpsc_cart object to have been loaded.

If a gateway needs to insert information into the form based on information about the cart there doesn't seem to be a clean way to do so. This filter makes the checkout form output be passed through a filter at the point it's rendered so that gateways can add additional information. 

It worksforme(TM) but could probably use a bit of sanity checking as I haven't had chance to test it comprehensively across all gateways.