Add initial support for speex RTP packetization (RFC 5574)

* libspeex audio codec is no longer considered unsupported
  when using rtp as output format.
* SDP rtpmap is added for speex payload, formatted according to RFC
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtpenc.c

@@ -75,6 +75,7 @@ static int is_supported(enum AVCodecID id)
     case AV_CODEC_ID_ADPCM_G722:
     case AV_CODEC_ID_ADPCM_G726:
     case AV_CODEC_ID_ILBC:
+    case AV_CODEC_ID_SPEEX:
         return 1;
     default:
         return 0;

libavformat/sdp.c

@@ -567,6 +567,10 @@ static char *sdp_write_media_attributes(char *buff, int size, AVCodecContext *c,
                                      payload_type, c->sample_rate,
                                      payload_type, c->block_align == 38 ? 20 : 30);
             break;
+        case AV_CODEC_ID_SPEEX:
+            av_strlcatf(buff, size, "a=rtpmap:%d speex/%d\r\n",
+                                     payload_type, c->sample_rate);
+
         default:
             /* Nothing special to do here... */
             break;