UI: Beginning of customizable settings, debug menu

src/duckstation/sdl_interface.h

@@ -87,4 +87,7 @@ private:
   u32 m_last_frame_number = 0;
   u32 m_last_internal_frame_number = 0;
   Timer m_fps_timer;
+
+  // UI options
+  bool m_show_gpu_statistics = false;
 };