LibAudio: Split Buffer.{h, cpp} into three files

The Buffer files had contained both the ResampleHelper and the
sample format utilities. Because the Buffer class (and its file) is
going to be deleted soon, this commit separates those two things into
their own files.

Userland/Libraries/LibAudio/Resampler.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Resampler.h"
+#include "Buffer.h"
+#include "Sample.h"
+
+namespace Audio {
+
+ErrorOr<NonnullRefPtr<Buffer>> resample_buffer(ResampleHelper<double>& resampler, Buffer const& to_resample)
+{
+    Vector<Sample> resampled;
+    resampled.ensure_capacity(to_resample.sample_count() * ceil_div(resampler.source(), resampler.target()));
+    for (size_t i = 0; i < static_cast<size_t>(to_resample.sample_count()); ++i) {
+        auto sample = to_resample.samples()[i];
+        resampler.process_sample(sample.left, sample.right);
+
+        while (resampler.read_sample(sample.left, sample.right))
+            resampled.append(sample);
+    }
+
+    return Buffer::create_with_samples(move(resampled));
+}
+
+}