Add initial libretro implementation

src/duckstation-libretro/libretro_host_interface.h

@@ -0,0 +1,57 @@
+#pragma once
+#include "core/host_interface.h"
+#include "core/system.h"
+#include "libretro.h"
+
+class LibretroHostInterface : public HostInterface
+{
+public:
+  LibretroHostInterface();
+  ~LibretroHostInterface() override;
+
+  static bool SetCoreOptions();
+
+  bool Initialize() override;
+  void Shutdown() override;
+
+  void ReportError(const char* message) override;
+  void ReportMessage(const char* message) override;
+  bool ConfirmMessage(const char* message) override;
+
+  const retro_hw_render_callback& GetHWRenderCallback() const { return m_hw_render_callback; }
+
+  // Called by frontend
+  void retro_get_system_av_info(struct retro_system_av_info* info);
+  bool retro_load_game(const struct retro_game_info* game);
+  void retro_run_frame();
+  unsigned retro_get_region();
+
+protected:
+  bool AcquireHostDisplay() override;
+  void ReleaseHostDisplay() override;
+  std::unique_ptr<AudioStream> CreateAudioStream(AudioBackend backend) override;
+
+private:
+  void LoadSettings();
+  void UpdateSettings();
+  void UpdateControllers();
+  void UpdateControllersDigitalController(u32 index);
+
+  // Hardware renderer setup.
+  bool RequestHardwareRendererContext();
+
+  static void HardwareRendererContextReset();
+  static void HardwareRendererContextDestroy();
+
+  retro_hw_render_callback m_hw_render_callback = {};
+};
+
+extern LibretroHostInterface g_libretro_host_interface;
+
+// libretro callbacks
+extern retro_environment_t g_retro_environment_callback;
+extern retro_video_refresh_t g_retro_video_refresh_callback;
+extern retro_audio_sample_t g_retro_audio_sample_callback;
+extern retro_audio_sample_batch_t g_retro_audio_sample_batch_callback;
+extern retro_input_poll_t g_retro_input_poll_callback;
+extern retro_input_state_t g_retro_input_state_callback;