Ambisonics not available

[chris-hld]

Opusfile fails to decode Ambisonics.
Both mapping families (2 and 3) are broken / not implemented.

With a third order Ambix file, the encoding works:

./../../opus-tools/opusenc --channels ambix opus_AmbixTCs.wav opus_ambi.opus    
Skipping chunk of type "fact", length 4
Skipping chunk of type "PEAK", length 136
Encoding using libopus 1.4-6-g9fc8fc4c (audio)
-----------------------------------------------------
   Input: 48 kHz, 16 channels
  Output: 16 channels (16 coupled)
          20ms packets, 768 kbit/s VBR
 Preskip: 312

Encoding complete                                
-----------------------------------------------------
       Encoded: 10.02 seconds
       Runtime: 1 second
                (10.02x realtime)
         Wrote: 1028095 bytes, 501 packets, 19 pages
       Bitrate: 815.996 kbit/s (without overhead)
 Instant rates: 12.4 to 1496.4 kbit/s
                (31 to 3741 bytes per packet)
      Overhead: 0.589% (container+metadata)

The bitstream seams also seems valid

./../../opus-tools/opusinfo opus_ambi.opus                                  
Processing file "opus_ambi.opus"...

New logical stream (#1, serial: 572bd79b): type opus
Encoded with libopus 1.4-6-g9fc8fc4c, libopusenc 0.2.1-16-ge4285b5
User comments section follows...
	ENCODER=opusenc from opus-tools 0.2-32-ge7f495e
	ENCODER_OPTIONS=--channels ambix
Opus stream 1:
	Pre-skip: 312
	Playback gain: 0 dB
	Channels: 16
	Original sample rate: 48000 Hz
	Streams: 8, Coupled: 8
	Channel Mapping Family: 3
	Demixing Matrix [16x16]:
	[  8192,      0,  -3413,  13829,      0,      0,  -8176,  -7075,  15617,      0,      0,      0,  11826,  -7890,  -9974,  16112]
	[  8192,  -9779,   3413,   9779, -15617,  -5003,  -8176,   5003,      0, -11393,  -9974,   5579, -11826,  -5579,      0, -11393]
	[  8192,   9779,   3413,  -9779, -15617,   5003,  -8176,  -5003,      0,  11393,  -9974,  -5579, -11826,   5579,      0,  11393]
	[  8192,   6263, -11359,   6263,   6406, -10664,   8208, -10664,      0,   2993, -13617,  10185,   -901,  10185,      0,  -2993]
	[  8192,   8857,  11359,      0,      0,  15081,   8208,      0,  -6406,  -4233,      0,  14403,    901,      0, -13617,      0]
	[  8192,      0,  11359,   8857,      0,      0,   8208,  15081,   6406,      0,      0,      0,    901,  14403,  13617,   4233]
	[  8192,   6263, -11359,  -6263,  -6406, -10664,   8208,  10664,      0,   2993,  13617,  10185,   -901, -10185,      0,   2993]
	[  8192,  13829,  -3413,      0,      0,  -7075,  -8176,      0, -15617, -16112,      0,  -7890,  11826,      0,   9974,      0]
	[  8192,   9779,   3413,   9779,  15617,   5003,  -8176,   5003,      0,  11393,   9974,  -5579, -11826,  -5579,      0, -11393]
	[  8192, -13829,  -3413,      0,      0,   7075,  -8176,      0, -15617,  16112,      0,   7890,  11826,      0,   9974,      0]
	[  8192,      0,  -3413, -13829,      0,      0,  -8176,   7075,  15617,      0,      0,      0,  11826,   7890,  -9974, -16112]
	[  8192,  -6263, -11359,   6263,  -6406,  10664,   8208, -10664,      0,  -2993,  13617, -10185,   -901,  10185,      0,  -2993]
	[  8192,      0,  11359,  -8857,      0,      0,   8208, -15081,   6406,      0,      0,      0,    901, -14403,  13617,  -4233]
	[  8192,  -8857,  11359,      0,      0, -15081,   8208,      0,  -6406,   4233,      0, -14403,    901,      0, -13617,      0]
	[  8192,  -6263, -11359,  -6263,   6406,  10664,   8208,  10664,      0,  -2993, -13617, -10185,   -901, -10185,      0,   2993]
	[  8192,  -9779,   3413,  -9779,  15617,  -5003,  -8176,  -5003,      0, -11393,   9974,   5579, -11826,   5579,      0,  11393]
	Packet duration:   20.0ms (max),   20.0ms (avg),   20.0ms (min)
	Page duration:    640.0ms (max),  589.4ms (avg),  500.0ms (min)
	Total data length: 1028095 bytes (overhead: 0.589%)
	Playback length: 0m:10.000s
	Average bitrate: 822.5 kbit/s, w/o overhead: 817.6 kbit/s
Logical stream 1 ended

However, decoding fails very quickly:

./../../opus-tools/opusdec opus_ambi.opus opus_ambi_dec.wav                
Failed to open 'opus_ambi.opus'.

This PR fixes at least Ambisonics mapping 2 #45

[chris-hld]

#45 now also fixes Ambisonics mapping 3 and mapping 255.

[rillian]

See also https://gitlab.xiph.org/xiph/opusfile/-/merge_requests/40