[flac-dev] WAVEFORMATEXTENSIBLE_CHANNEL_MASK is not described

[lvqcl]

An issue was raised at <http://forum.doom9.org/showthread.php?p=1728923#post1728923> -
FLAC uses WAVEFORMATEXTENSIBLE_CHANNEL_MASK tag to describe non-standard layout,
but it isn't mentioned anywhere in FLAC format.

Channel assignment is described at <https://xiph.org/flac/format.html#frame_header>:
"Where defined, the channel order follows SMPTE/ITU-R recommendations."

The undefined value (0000?) is not mentioned. And no mentions that FLAC can
store channel assignment in WAVEFORMATEXTENSIBLE_CHANNEL_MASK tag, and how and
when to use it.