Free-run the envelope if the sustain value is low enough

Value taken from Sforzando

src/sfizz/ADSREnvelope.cpp

@@ -52,7 +52,7 @@ void ADSREnvelope::reset(const EGDescription& desc, const Region& region, const
     sustainThreshold = this->sustain + config::virtuallyZero;
     shouldRelease = false;
     freeRunning = (
-        (this->sustain == Float(0.0))
+        (this->sustain <= Float(config::sustainFreeRunningThreshold))
         || (region.loopMode == LoopMode::one_shot && region.isOscillator())
     );
     currentValue = this->start;

src/sfizz/Config.h

@@ -167,6 +167,11 @@ namespace config {
      * @brief The smoothing time constant per "smooth" steps
      */
     constexpr float smoothTauPerStep { 3e-3 };
+    /**
+     * @brief If a value below this threshold is given to `ampeg_sustain`, the envelope will free-run
+     * and the voice will release itself at the end of the decay stage.
+     */
+    constexpr float sustainFreeRunningThreshold { 0.0032f };
 } // namespace config
 
 } // namespace sfz