lavf/matroskaenc: enable PGS merge auto bsf

PGS segments must be merged to one packet for muxing to mkv

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

configure

@@ -3435,7 +3435,7 @@ latm_muxer_select="aac_adtstoasc_bsf mpeg4audio"
 matroska_audio_muxer_select="matroska_muxer"
 matroska_demuxer_select="riffdec"
 matroska_demuxer_suggest="bzlib zlib"
-matroska_muxer_select="mpeg4audio riffenc vp9_superframe_bsf aac_adtstoasc_bsf"
+matroska_muxer_select="mpeg4audio riffenc aac_adtstoasc_bsf pgs_frame_merge_bsf vp9_superframe_bsf"
 mlp_demuxer_select="mlp_parser"
 mmf_muxer_select="riffenc"
 mov_demuxer_select="iso_media riffdec"

libavformat/matroskaenc.c

@@ -3172,6 +3172,9 @@ static int mkv_check_bitstream(AVFormatContext *s, AVStream *st,
             ret = ff_stream_add_bitstream_filter(st, "aac_adtstoasc", NULL);
     } else if (st->codecpar->codec_id == AV_CODEC_ID_VP9) {
         ret = ff_stream_add_bitstream_filter(st, "vp9_superframe", NULL);
+    } else if (CONFIG_MATROSKA_MUXER &&
+               st->codecpar->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE) {
+        ret = ff_stream_add_bitstream_filter(st, "pgs_frame_merge", NULL);
     }
 
     return ret;