mcd, add dr_mp3 playback support

platform/common/dr_libs

@@ -0,0 +1 @@
+Subproject commit 343aa923439e59e7a9f7726f70edc77a4500bdec

platform/common/mp3_drmp3.c

@@ -0,0 +1,65 @@
+/*
+ * MP3 decoding using dr_mp3
+ * (C) kub, 2020
+ *
+ * This work is licensed under the terms of MAME license.
+ * See COPYING file in the top-level directory.
+ */
+
+#include <stdio.h>
+
+#include <pico/pico_int.h>
+#define DR_MP3_IMPLEMENTATION
+#include "dr_libs/dr_mp3.h"
+#include "mp3.h"
+
+static drmp3dec mp3dec;
+static unsigned char mp3_input_buffer[2 * 1024];
+
+int mp3dec_start(FILE *f, int fpos_start)
+{
+	drmp3dec_init(&mp3dec);
+	return 0;
+}
+
+int mp3dec_decode(FILE *f, int *file_pos, int file_len)
+{
+	drmp3dec_frame_info info;
+	unsigned char *readPtr;
+	int bytesLeft;
+	int offset; // mp3 frame offset from readPtr
+	int len;
+	int retry = 3;
+
+	do
+	{
+		if (*file_pos >= file_len)
+			return 1; /* EOF, nothing to do */
+
+		fseek(f, *file_pos, SEEK_SET);
+		bytesLeft = fread(mp3_input_buffer, 1, sizeof(mp3_input_buffer), f);
+
+		offset = mp3_find_sync_word(mp3_input_buffer, bytesLeft);
+		if (offset < 0) {
+			lprintf("find_sync_word (%i/%i) err %i\n",
+				*file_pos, file_len, offset);
+			*file_pos = file_len;
+			return 1; // EOF
+		}
+		*file_pos += offset;
+		readPtr = mp3_input_buffer + offset;
+		bytesLeft -= offset;
+
+		len = drmp3dec_decode_frame(&mp3dec, readPtr, bytesLeft, cdda_out_buffer, &info);
+		if (len > 0)			// retrieved decoded data
+			*file_pos += info.frame_bytes;
+		else if (info.frame_bytes > 0)	// no output but input consumed?
+			*file_pos += 1;			// try to skip frame
+		else if (offset == 0)		// bad frame?
+			*file_pos += 1;			// try resyncing
+		// else				// truncated frame, try more data
+	}
+	while (len <= 0 && --retry > 0);
+
+	return len <= 0;
+}