setTainted(isTainted){
        this._isTainted = isTainted;
    }
Set the internal _isTainted flag for this TiledImage. Lazy loaded - not
checked each time a Tile is loaded, but can be set if a consumer of the
tiles (e.g. a Drawer) discovers a Tile to have tainted data so that further
checks are not needed and alternative rendering strategies can be used.
@private