Respect Window.AllowAltF4 For SDLGamePlatform (MonoGame#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;