explicitly handle contextLayer callback - may run faster in some scenarios

Author: archmoj

src/traces/parcoords/parcoords.js

@@ -456,12 +456,18 @@ module.exports = function(root, svg, parcoordsLineLayers, styledData, layout, ca
 
     parcoordsLineLayer
         .each(function(d) {
+
             if(d.viewModel) {
                 if(d.lineLayer) d.lineLayer.update(d);
                 else d.lineLayer = lineLayerMaker(this, d);
 
                 d.viewModel[d.key] = d.lineLayer;
-                d.lineLayer.render(d.viewModel.panels, true);
+
+                var setChanged = ((d.key) &&
+                    (((d.key !== 'contextLayer') || (callbacks)) || // unless there is callback on this line layer
+                    (!d.context))); // don't update background
+
+                d.lineLayer.render(d.viewModel.panels, setChanged);
             }
         });