adjustable portamento, fixed adsr

Source/Shifter.h

@@ -84,6 +84,11 @@ public:
     void RemoveMidiNote(int note);
 	void SetFormantPreserve(float val) { formant_preserve = val; }
     void SetAutoTuneSpeed(float val);
+    void SetPortamentoTime(float time) {
+        for (int i = 0; i < MAX_VOICES; ++i) {
+            voices[i].SetPortamentoTime(time);
+        }
+	}
 
     float out_midi = 40;