compute(x, y, R, callback) {
        //You can always see your own tile
        callback(x, y, 0, 1);
        for (let i = 0; i < OCTANTS.length; i++) {
            this._renderOctant(x, y, OCTANTS[i], R, callback);
        }
    }
Compute visibility for a 360-degree circle
@param {int} x
@param {int} y
@param {int} R Maximum visibility radius
@param {function} callback