fix: calculate normals without struct

inc/Filter.h

@@ -2,21 +2,16 @@
 #include "Effect.h"
 
 class Filter : public Effect {
-
   protected:
-    struct Normals {
-        float V;
-        float K;
-    };
-
     float m_freq;  // cutoff frequency
     float m_q;     // filter quantity (resonance)
     float m_order; // filter order (peakGain)
     /* todo: filter adsr */
-    float m_norm;
+    float m_norm, m_v, m_k;
     float m_a0, m_a1, m_a2, m_b1, m_b2;
     float m_z1, m_z2;
-    Normals calculate_normals();
+
+    void calculate_normals();
     virtual void calculate_coefficients(){};
 
   public: