headless: collapse headless window definition into a "Windows" folder, change GetWindowFlags to an abstract property.

src/Ryujinx/Headless/Windows/MetalWindow.cs

@@ -0,0 +1,47 @@
+using Ryujinx.Common.Configuration;
+using Ryujinx.Input.HLE;
+using Ryujinx.SDL2.Common;
+using SharpMetal.QuartzCore;
+using System.Runtime.Versioning;
+using static SDL2.SDL;
+
+namespace Ryujinx.Headless
+{
+    [SupportedOSPlatform("macos")]
+    class MetalWindow : WindowBase
+    {
+        private CAMetalLayer _caMetalLayer;
+
+        public CAMetalLayer GetLayer()
+        {
+            return _caMetalLayer;
+        }
+
+        public MetalWindow(
+            InputManager inputManager,
+            GraphicsDebugLevel glLogLevel,
+            AspectRatio aspectRatio,
+            bool enableMouse,
+            HideCursorMode hideCursorMode, 
+            bool ignoreControllerApplet)
+            : base(inputManager, glLogLevel, aspectRatio, enableMouse, hideCursorMode, ignoreControllerApplet) { }
+
+        public override SDL_WindowFlags WindowFlags => SDL_WindowFlags.SDL_WINDOW_METAL;
+
+        protected override void InitializeWindowRenderer()
+        {
+            void CreateLayer()
+            {
+                _caMetalLayer = new CAMetalLayer(SDL_Metal_GetLayer(SDL_Metal_CreateView(WindowHandle)));
+            }
+
+            SDL2Driver.MainThreadDispatcher?.Invoke(CreateLayer);
+        }
+
+        protected override void InitializeRenderer() { }
+
+        protected override void FinalizeWindowRenderer() { }
+
+        protected override void SwapBuffers() { }
+    }
+}