Fixes placeholders occasionally not rendering

Problem
----------------------------
Swiping and causing many pages to scroll, and then moving the mouse over the content while the content is scrolling would prevent the placeholders for the new pages from rendering.

This occurred since mousemove events would be added to the `TransfomrationQueue` after the dragend event. When the dragend event was processed, the queue would not be empty, and so it would wait to call `ScrollList.render()`. When the mousemove events process they do not trigger a render. So we'd end in a state where the `TransformationQueue` was drained, but the placeholders in view would not be rendered.

Solution
----------------------------
The solution is for the `TransformationQueue` to ignore mousemove events.

Testing
----------------------------
All unit tests should pass

```
git fetch
git checkout master
```

* Open the example, and make a large swipe or two to scroll past about 10 pages.
* Wiggle your mouse around the viewport until the content stops scrolling.
* The pages should not render.

```
git checkout fix_placeholders_occasionally_not_rendering
```

* Do the same actions.
* This time the content should render.


@timmccall-wf @tomconnell-wf @robbecker-wf @patkujawa-wf @todbachman-wf