code: function _runInDataframes (viewportExpressions, renderLayer, dataframes) {
      const processedFeaturesIDs = new Set(); // same feature can belong to multiple dataframes
      const viz = renderLayer.viz;
  
      const inViewportFeaturesIDs = new Set();
      dataframes.forEach(dataframe => {
          _runInDataframe(viz, viewportExpressions, dataframe, processedFeaturesIDs, inViewportFeaturesIDs);
      });
      return inViewportFeaturesIDs;
  }
docs: Run all viewport aggregations in the dataframes, and returns a list of featureIDs inside the
  viewport & not filtered out. That's a list of the features effectively included in the viewportExpressions run