[flac-dev] Define 6.1 and 7.1 channel mappings

[Ralph Giles]

The FLAC format specification never defined the semantics of 7- and
8-channel files, which has caused some pain for some years now.

Attached is a patch to define them. I don't know if this follows
"follows SMPTE/ITU-R recommendations," but it follows common tool
practice. I chose the set of surround speaker designations used by home
theatre systems, which is the same set used by the Vorbis and Opus mappings.

The ordering follows the WAVE file format, rather than the Vorbis order
of other Xiph codecs. This matches what flac has done for the existing
mappings, and it allegedly what current tools like libavformat are doing.

Please consider applying this patch to have a definitive statement on
the channel map for implementors to follow.

 -r

P.S. This doesn't update the flac command-line tool to enforce this
mapping. That needs to be done as well, along with test files.
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