add audio buffer class to remove std::span dependency

synth/ivoice.h

@@ -1,11 +1,14 @@
 #pragma once
-#include <span>
+
+#include "audio_buffer.h"
+#include <vector>
 
 namespace trnr {
 template <typename t_sample>
 struct ivoice {
 	virtual ~ivoice() = default;
-    virtual void process_samples(t_sample** _outputs, int _start_index, int _block_size, std::span<std::span<t_sample>> _modulators = {}) = 0;
+	virtual void process_samples(t_sample** _outputs, int _start_index, int _block_size,
+								 std::vector<audio_buffer<t_sample>> _modulators = {}) = 0;
 	virtual bool is_busy() = 0;
 	virtual void set_samplerate(double samplerate) = 0;
 	virtual void note_on(int _note, float _velocity) = 0;