h261dec: Fix context initialization sequence

ff_mpv_common_init sets s->context_initialized.

This fixes decoding of h261 in the cases where the demuxer
hasn't already set the frame size.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
rtpenc: Avoid brittle switch fallthroughs

Instead explicitly jump to the default case in the cases where
it is wanted, and avoid fallthrough between different codecs,
which could easily introduce bugs if people editing the code
aren't careful.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtp: Initial H.261 support

The packetizer only supports splitting at GOB headers - if
such aren't available frequently enough, it splits at any
random byte offset (not at a macroblock boundary either, which
would be allowed by the spec) and sends a payload header pretend
that it starts with a GOB header.

As long as a receiver doesn't try to handle such cases cleverly
but just drops broken frames, this shouldn't matter too much
in practice.

Signed-off-by: Martin Storsjö <martin@martin.st>
libavformat: Add a muxer wrapping mpegts encoding into RTP

Since this structurally is quite different from normal RTP
(multiple streams are muxed into one single mpegts stream,
which is packetized into one single RTP session), it is kept
as a separate muxer.

Since this structurally also behaves differently than normal
RTP, all of the other muxers that do chained RTP muxing
(rtsp, sap, mp4) would need to be updated similarly to handle
this - in particular, creating one single rtp_mpegts muxer
for the whole presentation instead of one rtp muxer per stream.

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Adjust the pts of new fragments similarly to what is done for dts

The pts and the corresponding duration is written in sidx
atoms, thus make sure these match up correctly.

Signed-off-by: Martin Storsjö <martin@martin.st>