demux_mkv: support ALAC

Test sample was produced with ffmpeg. Extradata handling closely follows
libavformat/matroskadec.c.

demux/codec_tags.c

@@ -104,6 +104,7 @@ static const struct mp_codec_tag mp_audio_codec_tags[] = {
     {0xfffe                   , "pcm"},
     // ------- internal mplayer FourCCs ------
     {MKTAG('O', 'p', 'u', 's'), "opus"}, // demux_mkv.c
+    {MKTAG('a', 'L', 'a', 'C'), "alac"}, // demux_mkv.c
     {MKTAG('S', 'a', 'd', 'x'), "adpcm_adx"},
     {MKTAG('A', 'M', 'V', 'A'), "adpcm_ima_amv"},
     {MKTAG('R', 'o', 'Q', 'A'), "roq_dpcm"},

demux/demux_mkv.c

@@ -1321,6 +1321,7 @@ static struct mkv_audio_tag {
     { MKV_A_WAVPACK,   0, mmioFOURCC('W', 'V', 'P', 'K') },
     { MKV_A_TRUEHD,    0, mmioFOURCC('T', 'R', 'H', 'D') },
     { MKV_A_FLAC,      0, mmioFOURCC('f', 'L', 'a', 'C') },
+    { MKV_A_ALAC,      0, mmioFOURCC('a', 'L', 'a', 'C') },
     { MKV_A_REAL28,    0, mmioFOURCC('2', '8', '_', '8') },
     { MKV_A_REALATRC,  0, mmioFOURCC('a', 't', 'r', 'c') },
     { MKV_A_REALCOOK,  0, mmioFOURCC('c', 'o', 'o', 'k') },
@@ -1549,6 +1550,16 @@ static int demux_mkv_open_audio(demuxer_t *demuxer, mkv_track_t *track,
             sh_a->codecdata_len = size;
             memcpy(sh_a->codecdata, ptr, size);
         }
+    } else if (!strcmp(track->codec_id, MKV_A_ALAC)) {
+        if (track->private_size && track->private_size < 10000000) {
+            sh_a->codecdata_len = track->private_size + 12;
+            sh_a->codecdata = malloc(sh_a->codecdata_len);
+            char *data = sh_a->codecdata;
+            AV_WB32(data + 0, sh_a->codecdata_len);
+            memcpy(data + 4, "alac", 4);
+            AV_WB32(data + 8, 0);
+            memcpy(data + 12, track->private_data, track->private_size);
+        }
     } else if (track->a_formattag == mmioFOURCC('W', 'V', 'P', 'K') ||
                track->a_formattag == mmioFOURCC('T', 'R', 'H', 'D')) {
     copy_private_data:

demux/matroska.h

@@ -52,6 +52,7 @@
 #define MKV_A_QDMC       "A_QUICKTIME/QDMC"
 #define MKV_A_QDMC2      "A_QUICKTIME/QDM2"
 #define MKV_A_FLAC       "A_FLAC"
+#define MKV_A_ALAC       "A_ALAC"
 #define MKV_A_TTA1       "A_TTA1"
 #define MKV_A_WAVPACK    "A_WAVPACK4"
 #define MKV_A_TRUEHD     "A_TRUEHD"