matroskaenc: make SSA default subtitle codec.

It's a better format and we have an encoder for it, so it doesn't fail
like CODEC_ID_TEXT.

libavformat/matroskaenc.c

@@ -1205,7 +1205,7 @@ AVOutputFormat ff_matroska_muxer = {
     mkv_write_trailer,
     .flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS,
     .codec_tag = (const AVCodecTag* const []){ff_codec_bmp_tags, ff_codec_wav_tags, 0},
-    .subtitle_codec = CODEC_ID_TEXT,
+    .subtitle_codec = CODEC_ID_SSA,
 };
 #endif