Respect -copyinkf for subtitles.

Necessary to make it possible to copy teletext streams
(trac issue #531).
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

ffmpeg.c

@@ -4263,6 +4263,8 @@ static OutputStream *new_subtitle_stream(OptionsContext *o, AVFormatContext *oc,
 
     subtitle_enc->codec_type = AVMEDIA_TYPE_SUBTITLE;
 
+    MATCH_PER_STREAM_OPT(copy_initial_nonkeyframes, i, ost->copy_initial_nonkeyframes, oc, st);
+
     return ost;
 }