Use the libspeexdsp resampler

AFAICT it runs a little slower? but it's fixed point, and has much
better understood audio characteristics.

lib/speexdsp/libspeexdsp/config.h

@@ -0,0 +1,13 @@
+// Copyright 2023 jacqueline <me@jacqueline.id.au>
+//
+// SPDX-License-Identifier: GPL-3.0-only
+
+#pragma once
+
+#define FIXED_POINT
+#define EXPORT
+#define STDC_HEADERS
+#define USE_SMALLFT
+
+#define DISABLE_ENCODER
+#define DISABLE_FLOAT_API