Add short paths for the smoothers

When the input is basically constant in particular

src/sfizz/OnePoleFilter.h

@@ -25,6 +25,8 @@ class OnePoleFilter {
 public:
     OnePoleFilter() = default;
 
+    Type current() const { return state; }
+
     void setGain(Type gain)
     {
         G = gain / (1 + gain);

src/sfizz/Smoothers.cpp

@@ -27,10 +27,25 @@ void Smoother::reset(float value)
 
 void Smoother::process(absl::Span<const float> input, absl::Span<float> output)
 {
-    if (smoothing)
+    CHECK_SPAN_SIZES(input, output);
+    if (input.size() == 0)
+        return;
+
+    const auto midValue = input[input.size() / 2];
+    const bool shortcut = (
+        input.front() == input.back()
+        && input.front() == midValue
+        && input.front() == current()
+    );
+
+    if (smoothing && !shortcut) {
         filter.processLowpass(input, output);
-    else
+    }
+    else if (input.data() != output.data()) {
         copy<float>(input, output);
+    } else {
+        // Nothing to do
+    }
 }
 
 }

src/sfizz/Smoothers.h

@@ -45,6 +45,7 @@ public:
      */
     void process(absl::Span<const float> input, absl::Span<float> output);
 
+    float current() const { return filter.current(); }
 private:
     bool smoothing { false };
     OnePoleFilter<float> filter {};