CODAP-1010 Make graph respond immediately to change in background color (#2266)

* [#CODAP-1010] Bug fix: The user can change the background color of a graph

* It turned out that the only problem was that the graph was not immediately responding to the change in background color. But it was already displaying the new color on redisplay. So the solution is to expand the accessor of the `mstReaction` installed in `Background` to include the `plotBackgroundColor` property.

* * Code review tweak

Author: bfinzer

v3/src/components/data-display/components/background.tsx

@@ -274,11 +274,11 @@ export const Background = forwardRef<SVGGElement | HTMLDivElement, IProps>((prop
     )
   }, [renderBackground, dataDisplayModel])
 
-  useEffect(function respondToChangeInImage() {
-    mstReaction(() => dataDisplayModel?.plotBackgroundImage,
+  useEffect(function respondToBackgroundOrImageChange() {
+    mstReaction(() => [dataDisplayModel?.plotBackgroundImage, dataDisplayModel?.plotBackgroundColor],
       () => {
         renderBackground()
-      }, {name: "renderBackground", fireImmediately: true}, dataDisplayModel
+      }, {name: "renderBackground", equals: comparer.structural, fireImmediately: true}, dataDisplayModel
     )
   }, [dataDisplayModel, renderBackground])