sum channels to avoid premature pause

[redFrik]

summing the sound channels (after rectification) makes sure that there's only a single DetectSilence and that no channel can pause the node prematurely.

this is a fix for a subtle bug in DirtPause. fixing it shouldn't affect sound except in some very rare corner cases. and then one would want the fix applied anyways i guess.

issue demonstrated here using rms but same problem can be found in all places where DirtPause is used. e.g. reverbs stopping early.

SuperDirt.start;
~d1 = ~dirt.orbits[0];
~dirt.startSendRMS;

//with default graceTime 4 the following should finish around 9
//i.e. dirt_rms node pauses itself and stop sending messages
(
var start= Main.elapsedTime;
OSCdef(\rms, {|msg, time| (time-start).postln}, "/rms");
~d1.((sound: 'tutorial2', dur: 5, pan: 0.5));
)


//same code panned hard right.
//note: it will finish early - around 8!
(
var start= Main.elapsedTime;
OSCdef(\rms, {|msg, time| (time-start).postln}, "/rms");
~d1.((sound: 'tutorial2', dur: 5, pan: 1));
)

the silent left channel stopped dirt_rms node sending 1sec early.
with the proposed fix the two examples both hold the full duration.