Another proposed fix for #1302

I believe the unload and destroy events are being called on the Tile, but it's not deleting itself when it's a singleTile. Adding this isLoading set, erases correctly the Tile on a singleTile case, without putting other flags in general code of Tile. Please @ahocevar review if you think this might be more cleaner. This way we still use the TileManager for singleTile layers.