Add initial libretro implementation

src/duckstation-libretro/libretro_host_display.h

@@ -0,0 +1,30 @@
+#pragma once
+#include "core/host_display.h"
+#include <memory>
+
+class LibretroHostDisplay final : public HostDisplay
+{
+public:
+  LibretroHostDisplay();
+  ~LibretroHostDisplay();
+
+  RenderAPI GetRenderAPI() const override;
+  void* GetRenderDevice() const override;
+  void* GetRenderContext() const override;
+  void WindowResized(s32 new_window_width, s32 new_window_height) override;
+
+  std::unique_ptr<HostDisplayTexture> CreateTexture(u32 width, u32 height, const void* data, u32 data_stride,
+                                                    bool dynamic) override;
+  void UpdateTexture(HostDisplayTexture* texture, u32 x, u32 y, u32 width, u32 height, const void* data,
+                     u32 data_stride) override;
+  bool DownloadTexture(const void* texture_handle, u32 x, u32 y, u32 width, u32 height, void* out_data,
+                       u32 out_data_stride) override;
+
+  void SetVSync(bool enabled) override;
+
+  void Render() override;
+
+private:
+  s32 m_last_display_width = -1;
+  s32 m_last_display_height = -1;
+};