rtpenc: Set a default video codec

avconv doesn't map video streams to a muxer without specifying a
manual stream mapping if the default video codec is CODEC_ID_NONE.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc.c

@@ -465,7 +465,7 @@ AVOutputFormat ff_rtp_muxer = {
     .long_name         = NULL_IF_CONFIG_SMALL("RTP output format"),
     .priv_data_size    = sizeof(RTPMuxContext),
     .audio_codec       = CODEC_ID_PCM_MULAW,
-    .video_codec       = CODEC_ID_NONE,
+    .video_codec       = CODEC_ID_MPEG4,
     .write_header      = rtp_write_header,
     .write_packet      = rtp_write_packet,
     .write_trailer     = rtp_write_trailer,