demux_mkv: support dirac in mkv

Nobody uses this, and this is an absolute waste of time. Even the user
who reported this turned out to have produced a sample manually.

Sample produced with:

wget http://diracvideo.org/download/test-streams/raw/vts/vts.LD-8Mb.drc
mkvmerge -o dirac.mkv vts.LD-8Mb.drc

mkvmerge writes a sort of broken aspect ratio. libavformat interprets it
as 1:1 PAR, while demux_mkv thinks this is a 1:1 DAR. Maybe libavformat
is more correct here.

demux/demux_mkv.c

@@ -1119,6 +1119,7 @@ static const videocodec_info_t vinfo[] = {
     {MKV_V_THEORA,    mmioFOURCC('t', 'h', 'e', 'o'), 1},
     {MKV_V_VP8,       mmioFOURCC('V', 'P', '8', '0'), 0},
     {MKV_V_VP9,       mmioFOURCC('V', 'P', '9', '0'), 0},
+    {MKV_V_DIRAC,     mmioFOURCC('d', 'r', 'a', 'c'), 0},
     {NULL, 0, 0}
 };
 

demux/matroska.h

@@ -78,6 +78,7 @@
 #define MKV_V_VP9        "V_VP9"
 #define MKV_V_MJPEG      "V_MJPEG"
 #define MKV_V_UNCOMPRESSED "V_UNCOMPRESSED"
+#define MKV_V_DIRAC      "V_DIRAC"
 
 #define MKV_S_TEXTASCII  "S_TEXT/ASCII"
 #define MKV_S_TEXTUTF8   "S_TEXT/UTF8"