fix gtk3 default settings

src/Gtk/Avalonia.Gtk3/Gtk3Platform.cs

@@ -148,8 +148,8 @@ namespace Avalonia.Gtk3
 
     public class Gtk3PlatformOptions
     {
-        public bool? UseDeferredRendering { get; set; } = true;
-        public bool? UseGpuAcceleration { get; set; };
+        public bool? UseDeferredRendering { get; set; }
+        public bool? UseGpuAcceleration { get; set; }
         public ICustomGtk3NativeLibraryResolver CustomResolver { get; set; }
     }
 }