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 to ignore mousemove events while the Awesomemap is
being transformed. There should be no need to handle them at this
time anyway.

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 
Here's what was going on:

`TransformationQueue.enqueue` would be invoked with mousemove and ignore it. `_queue` would be empty.
Then, `TransformationQueue.processEvents` would be invoked, and set `_processing` to true, and complete.
Later, a drag would be enqueued, and processEvents invoked again, but it would bail since `_processing` was true.

The fix is to set _processing to true when an empty queue is processed. We can safely ignore any mousemove events that get pushed into the queue.