_renderOctant(x, y, octant, R, callback) {
        //Radius incremented by 1 to provide same coverage area as other shadowcasting radiuses
        this._castVisibility(x, y, 1, 1.0, 0.0, R + 1, octant[0], octant[1], octant[2], octant[3], callback);
    }
Render one octant (45-degree arc) of the viewshed
@param {int} x
@param {int} y
@param {int} octant Octant to be rendered
@param {int} R Maximum visibility radius
@param {function} callback