avcodec: Add a RFC 3389 comfort noise codec

This isn't too useful as a normal codec, but can be used in
voip style applications. The decoder updates the noise
generator parameters when a packet is given to it for decoding,
but if called with an empty packet, it generates more noise
according to the last parameters.

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

libavcodec/allcodecs.c

@@ -91,6 +91,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER (CINEPAK, cinepak);
     REGISTER_ENCDEC  (CLJR, cljr);
     REGISTER_DECODER (CLLC, cllc);
+    REGISTER_ENCDEC  (COMFORTNOISE, comfortnoise);
     REGISTER_DECODER (CSCD, cscd);
     REGISTER_DECODER (CYUV, cyuv);
     REGISTER_DECODER (DFA, dfa);