[labs/virtualizer] `virtualize` directive only re-renders children if `items` or `renderItem` have changed

Description

When using the virtualize directive, children fail to re-render in response to certain types of data changes.

Specifically, children are only re-rendered if data within the items array changes (or if the renderItem template itself is replaced). This means that any changes to data stored outside the items array will fail to trigger re-rendering, even if logic or bindings in the renderItem template refer to the changed data.

Steps to Reproduce

1. Go to the demo link
2. Click “update selected” button
3. See that selected changes in the second list, but not in the first.

Live Reproduction Link

Demo

Expected Results

selected should change in both lists.

cc: @Westbrook

[usergenic]

@graynorton I’ll update and include it in first round of functional tests this week.

[usergenic]

Yay! I fixed the test fixture to demonstrate the noChange behavior and have updated the code in #3133 to return this.render(). Will get this through and release next. Thanks for your patience 😃