a simple fold-back distortion filter.
if the signal exceeds the given threshold-level, it mirrors at the positive/negative
threshold-border as long as the singal lies in the legal range (-threshold..+threshold).
there is no range limit, so inputs doesn't need to be in -1..+1 scale.
threshold should be >0
depending on use (low thresholds) it makes sense to rescale the input to full amplitude
performs approximately the following code
(just without the loop)
while (in>threshold || in<-threshold)
// mirror at positive threshold
if (in>threshold) in= threshold - (in-threshold);
// mirror at negative threshold
if (in<-threshold) in= -threshold + (-threshold-in);