lavf/riff: Support decoding Ambisonic wav files.

Reported and tested by Andy Furniss, adf d lists at gmail

libavformat/riff.h

@@ -107,6 +107,8 @@ extern const AVCodecGuid ff_codec_wav_guids[];
 
 #define FF_MEDIASUBTYPE_BASE_GUID \
     0x00, 0x00, 0x10, 0x00, 0x80, 0x00, 0x00, 0xAA, 0x00, 0x38, 0x9B, 0x71
+#define FF_AMBISONIC_BASE_GUID \
+    0x21, 0x07, 0xD3, 0x11, 0x86, 0x44, 0xC8, 0xC1, 0xCA, 0x00, 0x00, 0x00
 
 static av_always_inline int ff_guidcmp(const void *g1, const void *g2)
 {

libavformat/riffdec.c

@@ -69,6 +69,8 @@ static void parse_waveformatex(AVIOContext *pb, AVCodecContext *c)
 
     ff_get_guid(pb, &subformat);
     if (!memcmp(subformat + 4,
+                (const uint8_t[]){ FF_AMBISONIC_BASE_GUID }, 12) ||
+        !memcmp(subformat + 4,
                 (const uint8_t[]){ FF_MEDIASUBTYPE_BASE_GUID }, 12)) {
         c->codec_tag = AV_RL32(subformat);
         c->codec_id  = ff_wav_codec_get_id(c->codec_tag,