Smooth parameter changes, crossfade filter type, add note fade-in and soft limiter

Source/SynthEngine.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <JuceHeader.h>
+#include <array>
 #include "SynthParams.h"
 #include "Presets.h"
 
@@ -54,6 +55,7 @@ private:
         double env = 0.0;
         int    envState = 0;
         int    age = 0;
+        int    fadeLen = 32;
         double ic1eq = 0.0, ic2eq = 0.0;
     };
 
@@ -101,6 +103,10 @@ private:
 
     int currentPresetIndex = 0;
     mutable juce::SpinLock paramLock;
+
+    std::array<float, kNumParams> smooth;
+    float smCoef = 1.0f;
+    float ftA = 1.0f, ftB = 0.0f, ftC = 0.0f;
 };
 
 } // namespace sm26