code: function startPolygon(coordCurr, normPrev, normNext, join_type, context){
      // If polygon starts on a tile boundary, don't add a join
      if (join_type === undefined || isCoordOutsideTile(coordCurr)) {
          addVertex(coordCurr, normNext, normNext, 1, 0, context, 1);
          addVertex(coordCurr, normNext, normNext, 0, 0, context, -1);
      }
      else {
          // If polygon starts within a tile, add a join
          var v = 0;
          if (join_type === JOIN_TYPE.miter) {
              addMiter(v, coordCurr, normPrev, normNext, context.miter_len_sq, true, context);
          }
          else {
              addJoin(join_type, v, coordCurr, normPrev, normNext, true, context);
          }
      }
  }
docs: Begin a polygon with a join connecting to the last segment (if valid join-type specified)