BhobUGens: use standard panning behavior for TGrains2, TGrains3

[nhthn]

fix #81. panning behavior was standardized in core at supercollider/supercollider#2136

[nhthn]

test case:

b = Buffer.read(s, Platform.resourceDir +/+ "sounds/a11wlk01.wav");

{ TGrains2.ar(2, Impulse.kr(100), b, dur: 0.05, pan: Line.kr(-2, 2, 1), att: 0.01, dec: 0.01) }.plot(1.0)

(
var y, h;
c = Buffer.alloc(s, s.sampleRate * 0.2);
y = Signal.hanningWindow(s.sampleRate * 0.4); // twice the size of our buffer, 'cause we only use 1/2
h = Array.fill(s.sampleRate * 0.2, {|i| y[i]});
c.loadCollection(h);
)

{ TGrains3.ar(2, Impulse.kr(100), b, dur: 0.05, pan: Line.kr(-2, 2, 1), att: 0.01, dec: 0.01, window: c) }.plot(1.0)

[bagong]

Fine on Windows! Thanks!