Respect Window.AllowAltF4 For SDLGamePlatform (#8248)

When checking the exit condition of `_isExiting > 0` an additional check is now made to determine if the exit should occur.  This check for the ALT+F4 keyboard combination, and if found, returns the state of `Window.AllowAltF4`; otherwise it return `true`.

MonoGame.Framework/Platform/SDL/SDLGamePlatform.cs

@@ -93,11 +93,27 @@ public override void RunLoop()
  Threading.Run();
  GraphicsDevice.DisposeContexts();
 
- if (_isExiting > 0)
+ if (_isExiting > 0 && ShouldExit())
+ {
  break;
+ }
+ else
+ {
+ _isExiting = 0;
+ }
  }
  }
 
+ private bool ShouldExit()
+ {
+ if(_keys.Contains(Keys.F4) && (_keys.Contains(Keys.LeftAlt) || _keys.Contains(Keys.RightAlt)))
+ {
+ return Window.AllowAltF4;
+ }
+
+ return true;
+ }
+
  private void SdlRunLoop()
  {
  Sdl.Event ev;