Support H.264 SEI recovery points.

H.264 SEI recovery points are put at frames in the stream that are not
(necessarily) keyframes, but that mark “if you decode the next N frames, you
will have a [perfectly or approximately] valid picture no matter what your
starting point was”. In particular, this is needed to decode streams encoded
with Periodic Intra Refresh (e.g. --sout-x264-intra-refresh true), at least if
you don't see the beginning of the stream, e.g. tuning into a multicast stream.
This may also help with some kinds of streams from AVCHD cameras that use
similar techniques.

One could argue that this functionality should live inside libavcodec instead,
but given that VLC does its own H.264 depacketization, this seems to be the
best place. I've tested it with streaming over UDP, and it seems to work fine.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>