Ring modulator rework:
- we now accurately (I hope) reproduce an integer overflow exploited by the distortion guitar patch (TFX, Inferno - Planet 2), thanks to Skatoony http://www.vogons.org/viewtopic.php?f=40&t=27560&start=620#p302071
- ring modulation model is refined from analysing the overflow
- added a comment regarding similar overflow with panning