MipMap, filtering, texture format support for 3D model loaders

Adding ModelLoader.ModelParameters to support TextureParameter, MipMapping and Filtering. Please note that all these settings are the same for all model textures (in case it has several). Also my patch does not check if the selected texture format is compatible with transparent materials if any.

After the path one could set the desired parameters prior to call assetManager.load, like
        ModelLoader.ModelParameters mp = new ModelLoader.ModelParameters();
        mp.textureParameter.genMipMaps = true;
        mp.textureParameter.format = Pixmap.Format.RGB565;
        mp.textureParameter.minFilter = Texture.TextureFilter.MipMapLinearNearest;
        mp.textureParameter.magFilter = Texture.TextureFilter.Linear;
        assetManager.load("model.g3db", Model.class, mp);

Or even we can set such parameters as default in ModelParameters constructor if one wish.