Added UI thread rendering mode for X11. (#14856)

Allows using UI thread render timer for X11. Makes sense for low-end X11 devices that previously used immediate renderer.

src/Avalonia.X11/X11Platform.cs

@@ -70,10 +70,14 @@ namespace Avalonia.X11
             if (options.UseDBusMenu)
                 DBusHelper.TryInitialize();
 
+            IRenderTimer timer = options.ShouldRenderOnUIThread
+               ? new UiThreadRenderTimer(60)
+               : new SleepLoopRenderTimer(60);
+
             AvaloniaLocator.CurrentMutable.BindToSelf(this)
                 .Bind<IWindowingPlatform>().ToConstant(this)
                 .Bind<IDispatcherImpl>().ToConstant(new X11PlatformThreading(this))
-                .Bind<IRenderTimer>().ToConstant(new SleepLoopRenderTimer(60))
+                .Bind<IRenderTimer>().ToConstant(timer)
                 .Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration(KeyModifiers.Control))
                 .Bind<IKeyboardDevice>().ToFunc(() => KeyboardDevice)
                 .Bind<ICursorFactory>().ToConstant(new X11CursorFactory(Display))
@@ -303,7 +307,14 @@ namespace Avalonia
         /// </remarks>
         public bool EnableSessionManagement { get; set; } = 
             Environment.GetEnvironmentVariable("AVALONIA_X11_USE_SESSION_MANAGEMENT") != "0";
-        
+
+        /// <summary>
+        /// Render directly on the UI thread instead of using a dedicated render thread.
+        /// This can be usable if your device don't have multiple cores to begin with.
+        /// This setting is false by default.
+        /// </summary>
+        public bool ShouldRenderOnUIThread { get; set; }
+
         public IList<GlVersion> GlProfiles { get; set; } = new List<GlVersion>
         {
             new GlVersion(GlProfileType.OpenGL, 4, 0),