rendering performance improvements (#968)

* freeze objects

* component rendering perf work

* use PureComponent where safe

* remove obsolete WorldUtil code

* make brushable histogram a pure component

client/src/components/brushableHistogram/index.js

@@ -13,15 +13,21 @@ import * as globals from "../../globals";
 import actions from "../../actions";
 import { makeContinuousDimensionName } from "../../util/nameCreators";
 
-@connect(state => ({
-  world: state.world,
-  scatterplotXXaccessor: state.controls.scatterplotXXaccessor,
-  scatterplotYYaccessor: state.controls.scatterplotYYaccessor,
-  continuousSelection: state.continuousSelection,
-  differential: state.differential,
-  colorAccessor: state.colors.colorAccessor
-}))
-class HistogramBrush extends React.Component {
+@connect((state, ownProps) => {
+  const { isObs, isUserDefined, isDiffExp, field } = ownProps;
+  const myName = makeContinuousDimensionName(
+    { isObs, isUserDefined, isDiffExp },
+    field
+  );
+  return {
+    world: state.world,
+    scatterplotXXaccessor: state.controls.scatterplotXXaccessor,
+    scatterplotYYaccessor: state.controls.scatterplotYYaccessor,
+    continuousSelectionRange: state.continuousSelection[myName],
+    colorAccessor: state.colors.colorAccessor
+  };
+})
+class HistogramBrush extends React.PureComponent {
   static getColumn(world, field, clipped = true) {
     /*
     Return the underlying Dataframe column for our field.   By default,
@@ -84,7 +90,7 @@ class HistogramBrush extends React.Component {
   }
 
   componentDidUpdate(prevProps) {
-    const { field, world, continuousSelection } = this.props;
+    const { field, world } = this.props;
     const { x, y, bins, svgRef } = this._histogram;
     let { brushXselection, brushX } = this.state;
     let forceBrushUpdate = false;
@@ -112,16 +118,8 @@ class HistogramBrush extends React.Component {
     if the selection has changed, ensure that the brush correctly reflects
     the underlying selection.
     */
-    if (
-      forceBrushUpdate ||
-      continuousSelection !== prevProps.continuousSelection
-    ) {
-      const { isObs, isUserDefined, isDiffExp } = this.props;
-      const myName = makeContinuousDimensionName(
-        { isObs, isUserDefined, isDiffExp },
-        field
-      );
-      const range = continuousSelection[myName];
+    const { continuousSelectionRange: range } = this.props;
+    if (forceBrushUpdate || range !== prevProps.continuousSelectionRange) {
       if (brushXselection) {
         const selection = d3.brushSelection(brushXselection.node());
         if (!range && selection) {
@@ -417,8 +415,8 @@ class HistogramBrush extends React.Component {
           isDiffExp
             ? "histogram-diffexp"
             : isUserDefined
-              ? "histogram-user-gene"
-              : "histogram-continuous-metadata"
+            ? "histogram-user-gene"
+            : "histogram-continuous-metadata"
         }
         style={{
           padding: globals.leftSidebarSectionPadding,