aviobuf: Partial support for reading in read/write contexts

So far, aviocontexts are used either in pure-read or pure-write
mode - full read/write mode doesn't work well (and implementing it
is a much larger, not totally trivial change).

This patch allows using avio_read and ffio_read_partial on
read/write aviocontexts, where the read operations are passed
through directly unbuffered, while writes are buffered as usual.

This is enough to support the operations needed by packet based
data transfer like in udp/rtp, where aviocontext is the only
public API for hooking up custom IO.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpdec: Move the URLContext used for RTCP RR out from the context, to a parameter

Signed-off-by: Martin Storsjö <martin@martin.st>
rtsp: Add support for depacketizing RTP data via custom IO

To use this, set sdpflags=custom_io to the sdp demuxer. During
the avformat_open_input call, the SDP is read from the AVFormatContext
AVIOContext (ctx->pb) - after the avformat_open_input call,
during the av_read_frame() calls, the same ctx->pb is used for reading
packets (and sending back RTCP RR packets).

Normally, one would use this with a read-only AVIOContext for the
SDP during the avformat_open_input call, then close that one and
replace it with a read-write one for the packets after the
avformat_open_input call has returned.

This allows using the RTP depacketizers as "pure" demuxers, without
having them tied to the libavformat network IO.

Signed-off-by: Martin Storsjö <martin@martin.st>
aviobuf: Discard old buffered, previously read data in ffio_read_partial

This makes RTP custom IO work properly with pure read-only
AVIOContexts as well.

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