Fix video player disposed after foreground load

Author: neon-nyan

BackgroundTest/CustomControl/LayeredBackgroundImage/LayeredBackgroundImage.Events.Loaders.cs

@@ -174,7 +174,10 @@ private async void LoadFromSourceAsyncDetached(
                 return;
             }
 
-            DisposeVideoPlayer();
+            if (grid.Name.StartsWith("Background"))
+            {
+                DisposeVideoPlayer();
+            }
 
             object? source = GetValue(sourceProperty);
             if (source is null)