oggdec: Parse theora headers since ogg might not mark keyframes

Fixes issue746

Originally committed as revision 22214 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/oggparsetheora.c

@@ -106,6 +106,7 @@ theora_header (AVFormatContext * s, int idx)
 
         st->codec->codec_type = CODEC_TYPE_VIDEO;
         st->codec->codec_id = CODEC_ID_THEORA;
+        st->need_parsing = AVSTREAM_PARSE_HEADERS;
 
     } else if (os->buf[os->pstart] == 0x83) {
         vorbis_comment (s, os->buf + os->pstart + 7, os->psize - 8);