Order entity in form_state/checkout pane forms is not the same as the one in checkout pane validate/submit callbacks

We have multiple checkout panes that allow to select additional options which are added as line items to the order.

In case of validation errors, we were seeing strange behavior where the line item changes that we made in a submit callback were not there when the checkout form callback was called again.

After a lot of debugging, we found out that the reason for this is in commerce_checkout_order_validate() in combination with the form cache, commerce_order_load() is used to make sure that it has an up to date order object before it starts to validate and submit the various checkout panes. As the form has been submitted, it does not rebuild the form and therefor does not call the form build functions but directly invokes validate, still having the order object from the form_state cache in $form_state.

In case of a validation error, it rebuilds the form using the defined arguments in $form_state['build_info'], which still is the old, cached order object and in commerce_checkout_form(), that is assigned to $form_state again. So the checkout pane forms are called with the old commerce order and sad things happen if they try to do stuff with that (e.g. trying to load line items which were deleted).

In either of those two locations, we need to ensure that we do have the correct, up to date order object. Either by doing $form_state['build_info']['args'][0] = $order in validate after loading it or by doing a commerce_order_load($order->order_id) in commerce_checkout_form() (which is free on normal calls as it's already in the static cache).