Bug #9365: multipart/alternative display fix

Bug #9365: multipart/alternative display fix

For undisplayable parts contained inside the alternative multipart that
is ultimately displayed, mark these parts as not handled by the
multipart/alternative driver.  Later, these parts will be rendered with
its native driver instead.

We thus need to go through and build all part data first before going
back and putting the final output together (since parts are no longer
rendered in sequential order).