make sound work

lib/minigb_apu/minigb_apu.h

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+/**
+ * minigb_apu is released under the terms listed within the LICENSE file.
+ *
+ * minigb_apu emulates the audio processing unit (APU) of the Game Boy. This
+ * project is based on MiniGBS by Alex Baines: https://github.com/baines/MiniGBS
+ */
+
+#pragma once
+
+#include <stdint.h>
+
+#define AUDIO_SAMPLE_RATE	44100
+
+#define DMG_CLOCK_FREQ		4194304.0
+#define SCREEN_REFRESH_CYCLES	70224.0
+#define VERTICAL_SYNC		(DMG_CLOCK_FREQ/SCREEN_REFRESH_CYCLES)
+
+#define AUDIO_SAMPLES		((unsigned)(AUDIO_SAMPLE_RATE / VERTICAL_SYNC))
+#define AUDIO_BUFFER_SIZE_BYTES (AUDIO_SAMPLES*4)
+
+/**
+ * Fill allocated buffer "data" with "len" number of 32-bit floating point
+ * samples (native endian order) in stereo interleaved format.
+ */
+void audio_callback(void *ptr, int16_t *data, size_t len);
+
+/**
+ * Read audio register at given address "addr".
+ */
+uint8_t audio_read(const uint16_t addr);
+
+/**
+ * Write "val" to audio register at given address "addr".
+ */
+void audio_write(const uint16_t addr, const uint8_t val);
+
+/**
+ * Initialise audio driver.
+ */
+void audio_init(void);