add global header support for subtitles encoding

Originally committed as revision 25742 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3577,6 +3577,10 @@ static void new_subtitle_stream(AVFormatContext *oc, int file_idx)
     if(subtitle_codec_tag)
         subtitle_enc->codec_tag= subtitle_codec_tag;
 
+    if (oc->oformat->flags & AVFMT_GLOBALHEADER) {
+        subtitle_enc->flags |= CODEC_FLAG_GLOBAL_HEADER;
+        avcodec_opts[AVMEDIA_TYPE_SUBTITLE]->flags |= CODEC_FLAG_GLOBAL_HEADER;
+    }
     if (subtitle_stream_copy) {
         st->stream_copy = 1;
     } else {