Similar to the listener, separate containers are provided atomically for the
mixer thread to apply updates without needing to block, and a free-list is used
to reuse container objects.
A couple things to note. First, the lock is still used when the effect state's
deviceUpdate method is called to prevent asynchronous calls to reset the device
from interfering. This can be fixed by using the list lock in ALc.c instead.
Secondly, old effect states aren't immediately deleted when the effect type
changes (the actual type, not just its properties). This is because the mixer
thread is intended to be real-time safe, and so can't be freeing anything. They
are cleared away when updates reuse the container they were kept in, and they
don't incur any extra processing cost, but there may be cases where the memory
is kept around until the effect slot is deleted.
Instead of looping over all the coefficients for each channel with multiplies,
when we know only one will have a non-0 factor for ambisonic mixing buffers,
just index the one with a non-0 factor.
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.
This is the same as allowed in the SF2 spec for its filter cutoff generator,
which can be used for a wah effect in MIDI. It makes sense to use a similar
range here.
It's now a low-pass filter with a cutoff that varies according to the input
signal amplitude.
There remains issues with apparent feedback in the resonant frequency with high
resonance values. The actual cutoff range for the filter is also a guess.
