fixed bug introduced with previous AssetManager change

At the same time, this change allows to change the texture settings while filling up the queue.
In the following sample, the first bunch of textures will use mipmaps, the second bunch won't.

    assets.useMipMaps = true;
    assets.enqueue(appDir.resolvePath("textures/with-mipmaps"));
    assets.useMipMaps = false;
    assets.enqueue(appDir.resolvePath("textures/without-mipmaps"));