Bug 1531833 - Only use two buffers instead of four in OpenSL ES backend for cubeb. r?achronop

This is what Oboe does.

Differential Revision: https://phabricator.services.mozilla.com/D21742

src/cubeb_opensl.c

@@ -84,7 +84,7 @@ struct cubeb {
 };
 
 #define NELEMS(A) (sizeof(A) / sizeof A[0])
-#define NBUFS 4
+#define NBUFS 2
 
 struct cubeb_stream {
   /* Note: Must match cubeb_stream layout in cubeb.c. */