Add op_get_last_page() for calculating duration.

Using op_get_prev_page_serial() meant if there were valid pages
 from another stream, we would re-scan them all repeatedly until we
 managed to back up, a page at a time, to a chunk with a page from
 the preferred stream with a valid granule position.
Breaking this case into its own function means we can guarantee we
 only scan each page once, and that we can stop as soon as we hit a
 page from a different link (in the unlikely event that the stream
 has no pages with valid timestamps).