We no longer return pictures chained using vlc_picture_chain_AppendChain().

The original code was returning the source picture, with a few metadata changed
and a secondary picture attached.

The secondary picture, with a different date, is prepared during the drain. We
keep a reference to the primary picture while waiting for the drain. This kept
picture is released during a Flush or on Close.

The last_pts that was used locally when chaining picture is kept as
prev_last_pts to generate the secondary picture(s) during the drain.