Including C headers inside of `extern "C"` breaks use from C++. Hoist
the includes of standard C headers above the block so we don't try
to mangle the stdlib.
I initially tried to scope this with a targeted change but it's too
hard to do correctly that way. This way, we avoid whack-a-mole.
Firefox is working around this in their e21461b7b8b39cc31ba53c47d4f6f310c673ff2f
commit.
Bug: https://bugzilla.mozilla.org/1953080
Use a wrap around delay ringbuffer. We can then avoid some modulo
arithmetic and read more efficiently.
Also handle the delay convolver case better by reversing the taps and
reading the taps and delay buffer without extra overhead.
Add an option to do a hilbert transform on the generated rear channels
to do a 90 degree pahse shift on them. This can improve spacialization
of the rear channels.
See #861
