Add treshold knob to peak controller
This causes the peak controller to react only when the measured peaks are above the set treshold
Might be useful for finetuning your sidechains
The decay knob didn't actually do anything before. Now it's useful
together with newly introduced attack knob. One can do much better
sidechaining now without disturbing artifacts due to too responsive
volume changes.
(cherry picked from commit 9615ceabd5)
