This can make GCC pretty noisey, complaining "declaration does not declare
anything" for each static_assert, but it should still function on such older
compilers.
Apparently, 5.1 surround sound is supposed to use the "side" channels, not the
back channels, and we've been wrong this whole time. That means the "5.1 Side"
is actually the correct 5.1 setup, and using the back channels is anomalous.
Additionally, this means the 5.1 buffer format should also use the the side
channels instead of the back channels.
A final note: the 5.1 mixing coefficients are changed so both use the original
5.1 surround sound set (with the surround channels at +/-110 degrees). So the
only difference now between 5.1 "side" and 5.1 "back" is the channel labels.
I don't like this, but it's currently necessary. The problem is that the
ambisonics-based panning does not maintain consistent energy output, which
causes sounds mapped directly to an output channel to be louder compared to
when being panned. The inconcistent energy output is partly by design, as it's
trying to render a full 3D sound field and at least attempts to correct for
imbalanced speaker layouts.
This behavior better matches Creative's hardware drivers and Rapture3D's OpenAL
driver. A compatibility environment variable is provided to restore the old
no-op behavior for any app that behaves badly from this change (set
__ALSOFT_SUSPEND_CONTEXT to "ignore").
If too many apps have a problem with this, the default behavior may need to be
changed to ignore, with the env var providing an option to defer/batch instead.
Although it is more correct for preserving the apparent volume, the ambisonics-
based panning does not work on the same power scale, making it louder by
comparison.
