feat(filter): add filter bank and filter models

Source/FilterBank.h

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+#include "Filter.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Snapshot of the two-slot filter section.
+// ===========================================================================
+struct FilterBankParams
+{
+    bool  f1On = false, f2On = false;
+    int   f1Type = 0, f2Type = 0;
+    float f1Cutoff = 0.5f, f2Cutoff = 0.5f;
+    float f1Res = 0.0f, f2Res = 0.0f;
+    float f1Drive = 0.0f, f2Drive = 0.0f;
+    float f1Key = 0.0f, f2Key = 0.0f;
+    int   f1Slope = 2, f2Slope = 2;
+    int   route = 0;
+    float mix   = 0.5f;
+    float out   = 1.0f;
+};
+
+// ===========================================================================
+// Two filter slots with serial / parallel / split routing, processed stereo.
+// ===========================================================================
+class FilterBank
+{
+public:
+    void prepare (double sampleRate, int maxBlockSize);
+    void reset();
+
+    void process (float* l, float* r, int numSamples, const FilterBankParams& p, float noteHz) noexcept;
+
+private:
+    Filter f1L, f1R, f2L, f2R;
+
+    static void applySlot (Filter& f, float* buf, int n, bool on, int type, float cutoff,
+                           float res, float drive, float key, int slope, float noteHz) noexcept;
+};
+
+} // namespace serum