Enable native windowing system settings with SDL platform

Source/Editor/Modules/UIModule.cs

@@ -381,7 +381,7 @@ namespace FlaxEditor.Modules
             Editor.Options.OptionsChanged += OnOptionsChanged;
 
             // Add dummy control for drawing the main window borders if using a custom style
-#if PLATFORM_WINDOWS
+#if PLATFORM_WINDOWS || PLATFORM_SDL
             if (!Editor.Options.Options.Interface.UseNativeWindowSystem)
 #endif
             {

Source/Editor/Modules/WindowsModule.cs

@@ -760,8 +760,10 @@ namespace FlaxEditor.Modules
             {
                 settings.HasBorder = false;
 
+#if !PLATFORM_SDL
                 // Skip OS sizing frame and implement it using LeftButtonHit
                 settings.HasSizingFrame = false;
+#endif
             }
 #elif PLATFORM_LINUX
             settings.HasBorder = false;