Add preliminary support for FLAC per the Matroska mapping.

The Matroska mapping is very simple: https://matroska.org/technical/specs/codecid/index.html

I haven't found more detailed documentation of the FLAC in Matroska mapping.
The Ogg mapping (https://xiph.org/flac/ogg_mapping.html) is also quite
simple due to the FLAC format being included inside the Ogg mapping.  Given
this, the FLAC in Matroska mapping is likely similar.

include/nestegg/nestegg.h

@@ -72,6 +72,7 @@ extern "C" {
 #define NESTEGG_CODEC_VP9     2       /**< Track uses Google On2 VP9 codec. */
 #define NESTEGG_CODEC_OPUS    3       /**< Track uses Xiph Opus codec. */
 #define NESTEGG_CODEC_AV1     4       /**< Track uses AOMedia AV1 codec. */
+#define NESTEGG_CODEC_FLAC    5       /**< Track uses Xiph FLAC codec. */
 #define NESTEGG_CODEC_UNKNOWN INT_MAX /**< Track uses unknown codec. */
 
 #define NESTEGG_VIDEO_MONO              0 /**< Track is mono video. */
@@ -276,6 +277,7 @@ int nestegg_track_type(nestegg * context, unsigned int track);
     @retval #NESTEGG_CODEC_AV1     Track codec is AV1.
     @retval #NESTEGG_CODEC_VORBIS  Track codec is Vorbis.
     @retval #NESTEGG_CODEC_OPUS    Track codec is Opus.
+    @retval #NESTEGG_CODEC_FLAC    Track codec is FLAC.
     @retval #NESTEGG_CODEC_UNKNOWN Track codec is unknown.
     @retval -1 Error. */
 int nestegg_track_codec_id(nestegg * context, unsigned int track);

src/nestegg.c

@@ -177,6 +177,7 @@ enum ebml_type_enum {
 #define TRACK_ID_AV1                "V_AV1"
 #define TRACK_ID_VORBIS             "A_VORBIS"
 #define TRACK_ID_OPUS               "A_OPUS"
+#define TRACK_ID_FLAC               "A_FLAC"
 
 /* Track Encryption */
 #define CONTENT_ENC_ALGO_AES        5
@@ -2482,6 +2483,9 @@ nestegg_track_codec_id(nestegg * ctx, unsigned int track)
   if (strcmp(codec_id, TRACK_ID_OPUS) == 0)
     return NESTEGG_CODEC_OPUS;
 
+  if (strcmp(codec_id, TRACK_ID_FLAC) == 0)
+    return NESTEGG_CODEC_FLAC;
+
   return NESTEGG_CODEC_UNKNOWN;
 }