Rewrite host GPU abstraction

- Don't have to repeat the same thing for 4 renderers.
 - Add native Metal renderer.

src/duckstation-nogui/win32_nogui_platform.h

@@ -23,6 +23,7 @@ public:
   void SetDefaultConfig(SettingsInterface& si) override;
 
   bool CreatePlatformWindow(std::string title) override;
+  bool HasPlatformWindow() const override;
   void DestroyPlatformWindow() override;
   std::optional<WindowInfo> GetPlatformWindowInfo() override;
   void SetPlatformWindowTitle(std::string title) override;