[Stk] Possible ADSR or Wurley bug

[Joseph Cooper]

The Wurley instrument, by default, sets its 4 ADSR envelopes with a
sustain level of zero.  The release rate of the ADSR is calculated as
a function of the sustain level, perhaps because of this previous
post: http://ccrma-mail.stanford.edu/pipermail/stk/2003-May/000104.html
In its current state, this means that releaseRate_ = 0 if
sustainLevel_ == 0.  Consequently, if you choose to release a note
before it has faded all the way to zero (i.e., with noteOff or
keyOff), then the envelope goes into release state, but never decays
from its current level.  The work-arounds are obvious, but it seemed
worth noting the phenomenon since it took me some code digging to
figure out why my notes didn't stop when I let go of the key.

jc