Add initial libretro implementation

src/duckstation-libretro/libretro_audio_stream.h

@@ -0,0 +1,21 @@
+#pragma once
+#include "common/audio_stream.h"
+#include <cstdint>
+#include <vector>
+
+class LibretroAudioStream final : public AudioStream
+{
+public:
+  LibretroAudioStream();
+  ~LibretroAudioStream();
+
+protected:
+  bool OpenDevice() override;
+  void PauseDevice(bool paused) override;
+  void CloseDevice() override;
+  void FramesAvailable() override;
+
+private:
+  // TODO: Optimize this buffer away.
+  std::vector<SampleType> m_output_buffer;
+};