channel layouts and up/down mixing

https://www.w3.org/Bugs/Public/show_bug.cgi?id=17379 has been open for
about six months. It really needs to be fixed, since without a fix, the
core functionality of mixing input nodes is undefined for all but a few
combinations of possible input/output channel counts.

The spec, and various comments made on the list, seem to suggest that for
"nonstandard" channel counts (everything except 1, 2, 4, and 6 presumably?)
the channel layouts could/should be left undefined/host-specific. But this
is untenable, since it means applications don't know what the channels mean
and UAs can't consistently upmix/downmix those channel counts.

If we can't agree on standard layouts for 3, 5 or > 6 channels, I think it
would be fine to just prohibit them for now. That constraint can be relaxed
later in a future version of the spec.

Rob
-- 
Jesus called them together and said, “You know that the rulers of the
Gentiles lord it over them, and their high officials exercise authority
over them. Not so with you. Instead, whoever wants to become great among
you must be your servant, and whoever wants to be first must be your
slave — just
as the Son of Man did not come to be served, but to serve, and to give his
life as a ransom for many.” [Matthew 20:25-28]