_clipWithPolygons (polygons, useSketch) {
        var context = this._getContext(useSketch);
        context.beginPath();
        for(const polygon of polygons){
            for(const [i, coord] of polygon.entries() ){
                context[i === 0 ? 'moveTo' : 'lineTo'](coord.x, coord.y);
            }
        }

        context.clip();
    }
This function will create multiple polygon paths on the drawing context by provided polygons,
then clip the context to the paths.
@private
@param {OpenSeadragon.Point[][]} polygons - an array of polygons. A polygon is an array of OpenSeadragon.Point
@param {Boolean} useSketch - Whether to use the sketch canvas or not.