Move shared frontend code from SDL to a seperate project

src/frontend-common/sdl_audio_stream.h

@@ -0,0 +1,20 @@
+#pragma once
+#include "common/audio_stream.h"
+#include <cstdint>
+
+class SDLAudioStream final : public AudioStream
+{
+public:
+  SDLAudioStream();
+  ~SDLAudioStream();
+
+protected:
+  bool OpenDevice() override;
+  void PauseDevice(bool paused) override;
+  void CloseDevice() override;
+  void BufferAvailable() override;
+
+  static void AudioCallback(void* userdata, uint8_t* stream, int len);
+
+  bool m_is_open = false;
+};