This solves the same problem as #27 but in a cleaner way. The previous
commits refactored the smoothing to make it possible to use the
calculations in plugin code to do the same thing as #27, but then I
realized that that doesn't make much sense since the time to decay into
complete silence isn't very meaningful for this kind of computation.
Only a couple of these functions would be needed during initialization.
In the next couple commits ProcessContext will get a way to access
auxiliary IO, so this really had to be separated.
