Add a helper to apply a gain span to effect inputs

Used for ramping out voice data

src/sfizz/Effects.cpp

@@ -109,6 +109,17 @@ void EffectBus::addToInputs(const float* const addInput[], float addGain, unsign
     }
 }
 
+void EffectBus::applyGain(const float* gain, unsigned nframes)
+{
+    if (!gain)
+        return;
+
+    absl::Span<const float> gainSpan { gain, nframes };
+    for (unsigned c = 0; c < EffectChannels; ++c) {
+        sfz::applyGain<float>(gainSpan, _inputs.getSpan(c));
+    }
+}
+
 void EffectBus::setSampleRate(double sampleRate)
 {
     for (const auto& effectPtr : _effects)

src/sfizz/Effects.h

@@ -136,6 +136,11 @@ public:
      */
     void addToInputs(const float* const addInput[], float addGain, unsigned nframes);
 
+    /**
+       @brief Apply a gain to the inputs
+     */
+    void applyGain(const float* gain, unsigned nframes);
+
     /**
        @brief Initializes all effects in the bus with the given sample rate.
      */