_clear(useSketch, bounds){
        var context = this._getContext(useSketch);
        if (bounds) {
            context.clearRect(bounds.x, bounds.y, bounds.width, bounds.height);
        } else {
            var canvas = context.canvas;
            context.clearRect(0, 0, canvas.width, canvas.height);
        }
    }
@private
@param {Boolean} useSketch Whether to clear sketch canvas or main canvas
@param {OpenSeadragon.Rect} [bounds] The rectangle to clear