Computes the tile file name and generates its contents if the file does not exist.

get_tile_file_name.rs

async fn get_tile_file_name(x: i32, y: i32, z:i32) -> String {
    let quad_key = Quadkey::tile_to_quadkey(x, y, z as usize);
    let mut file_name = format!("./tiles/{}/{}.png", z, quad_key);
    let path = Path::new(&file_name);

    if !path.exists() {
        let tile_quad_keys = get_tile_quad_keys(&quad_key).await;

        if tile_quad_keys.is_empty() {
            file_name = get_default_filename();
        } else {
            let tile_pixels = get_tile_pixels(&quad_key, tile_quad_keys);
            let range = get_level_range(quad_key.len() as i32 + 8).await;
            let picture = paint_tile(tile_pixels, 
                                     get_color_gradient(),
                                     range);
            let folder_name = format!("./tiles/{}", z);
            create_dir_all(&folder_name).unwrap();
            picture.save(&file_name).unwrap();
        }
    }
    file_name
}