LibAudio: Create a playback class with a PulseAudio implementation

This adds an abstract `Audio::PlaybackStream` class to allow cross-
platform audio playback to be done in an opaque manner by applications
in both Serenity and Lagom.

Currently, the only supported audio API is PulseAudio, but a Serenity
implementation should be added shortly as well.

Userland/Libraries/LibAudio/PlaybackStreamPulseAudio.h

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2023, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibAudio/PlaybackStream.h>
+#include <LibAudio/PulseAudioWrappers.h>
+
+namespace Audio {
+
+class PlaybackStreamPulseAudio final
+    : public PlaybackStream {
+public:
+    static ErrorOr<NonnullRefPtr<PlaybackStream>> create(OutputState initial_state, u32 sample_rate, u8 channels, u32 target_latency_ms, AudioDataRequestCallback&& data_request_callback);
+
+    virtual void set_underrun_callback(Function<void()>) override;
+
+    virtual NonnullRefPtr<Core::ThreadedPromise<Duration>> resume() override;
+    virtual NonnullRefPtr<Core::ThreadedPromise<void>> drain_buffer_and_suspend() override;
+    virtual NonnullRefPtr<Core::ThreadedPromise<void>> discard_buffer_and_suspend() override;
+
+    virtual ErrorOr<Duration> total_time_played() override;
+
+    virtual NonnullRefPtr<Core::ThreadedPromise<void>> set_volume(double) override;
+
+private:
+    // This struct is kept alive until the control thread exits to prevent a use-after-free without blocking on
+    // the UI thread.
+    class InternalState : public AtomicRefCounted<InternalState> {
+    public:
+        void set_thread(NonnullRefPtr<Threading::Thread> const&);
+
+        void set_stream(NonnullRefPtr<PulseAudioStream> const&);
+        RefPtr<PulseAudioStream> stream();
+
+        void enqueue(Function<void()>&&);
+        void thread_loop();
+        ErrorOr<void> check_is_running();
+        void exit();
+
+    private:
+        RefPtr<PulseAudioStream> m_stream { nullptr };
+
+        Queue<Function<void()>> m_tasks;
+        Threading::Mutex m_mutex;
+        Threading::ConditionVariable m_wake_condition { m_mutex };
+
+        Atomic<bool> m_exit { false };
+
+        RefPtr<Threading::Thread> m_thread { nullptr };
+    };
+
+    PlaybackStreamPulseAudio(NonnullRefPtr<InternalState>);
+    ~PlaybackStreamPulseAudio();
+
+    RefPtr<InternalState> m_state;
+};
+
+}