Output I2S audio with PIO/DMA

Requires a MAX98357A amplifier and a 2W 8ohms speaker

ext/minigb_apu/minigb_apu.h

@@ -9,19 +9,20 @@
 
 #include <stdint.h>
 
-#define AUDIO_SAMPLE_RATE	32768
+#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, uint8_t *data, int len);
+void audio_callback(void *ptr, int16_t *data, size_t len);
 
 /**
  * Read audio register at given address "addr".