rtpenc_chain: Use the original AVFormatContext for getting payload type

In ff_rtp_get_payload_type, the AVFormatContext is used for checking
whether the payload_type or rtpflags options are set. In rtpenc_chain,
the rtpctx struct is a newly initialized struct where no options have
been set yet, so no options can be fetched from there.

All muxers that internally chain rtp muxers have the "rtpflags" field
that allows passing such options on (which is how this worked before
8034130e06), so this works just as intended.

This makes it possible to produce H263 in RFC2190 format with chained
RTP muxers.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
