lavc: add a public API for parsing vorbis packets.

It is required by (at least) the ogg demuxer.

Mark the current semi-public apriv API for removal.

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:     2014-08-09
 
 API changes, most recent first:
 
+2014-11-xx - xxxxxxx - lavc 56.6.0 - vorbis_parser.h
+  Add a public API for parsing vorbis packets.
+
 2014-10-xx - xxxxxxx - lavu 54.5.0 - time.h
   Add av_gettime_relative().