Multichannel joint stereo simply interleaves stereo pairs (6ch: 2ch + 2ch + 2ch), so each pair is decoded separatedly.

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To test my changes, I converted examples to wav with ffmpeg.exe (old and new), and compared them to see they are byte-exact.

Regular 2ch files (JS and normal) were straightforward to test.

For multichannel, to check each JS pair is correctly decoded separatedly I did:
- manually demux 6ch.msf into 3 pairs and convert them (2ch_1.wav + 2ch_2.wav + 2ch_3.wav)
- convert the 6ch.msf file to wav (with my changes)
- manually demux the 6ch.wav into 3 pairs (6ch_d1.wav + 6ch_d2.wav + 6ch_d3.wav)
- compare each pair (ex. 2ch_3.wav vs 6ch_d3.wav): all pairs are byte-exact.

The new code just processes each JS pair separatedly, there are no algorithm changes.
It could be improved a bit but I'm not sure about typical styles.
I've only seen 6ch .MSF (probably the AT3 spec only supports 2ch audio).
Signed-off-by: bnnm <bananaman255@gmail.com>