Don't store windowed size when in fullscreen

WindowState can sometimes be stale as `storeWindowSizeToConfig()`
may be called from `HandleEventFromWatch()`.

osu.Framework/Platform/SDL3/SDL3Window_Windowing.cs

@@ -10,6 +10,7 @@
 using System.Linq;
 using osu.Framework.Bindables;
 using osu.Framework.Configuration;
+using osu.Framework.Extensions.EnumExtensions;
 using osu.Framework.Logging;
 using osuTK;
 using SDL;
@@ -785,9 +786,9 @@ private void storeWindowPositionToConfig()
         /// </remarks>
         private bool updatingWindowStateAndSize;
 
-        private void storeWindowSizeToConfig()
+        private unsafe void storeWindowSizeToConfig()
         {
-            if (WindowState != WindowState.Normal)
+            if (WindowState != WindowState.Normal || SDL_GetWindowFlags(SDLWindowHandle).HasFlagFast(SDL_WindowFlags.SDL_WINDOW_FULLSCREEN))
                 return;
 
             storingSizeToConfig = true;