Refactor most of the opening and scanning between sources

neotools/neoadpcmextract.h

@@ -2,7 +2,11 @@
 #define __NEOADPCMEXTRACT_H__
 
 #include <stdio.h>
+#include <stdint.h>
 
-int vgmExtractSamples(FILE* file);
+typedef struct { uint8_t* data; size_t size, reserved; } Buffer;
+
+int vgmReadSample(FILE* fin, Buffer* buf);
+int vgmScanSample(FILE* file);
 
 #endif//__NEOADPCMEXTRACT_H__