Fix Race Condition in Display#clear on IE10

There was a race condition in Display#clear on IE10
because we resize and then clear that causes the canvas
to not actually end up cleared.  Clearing the current
viewport first solves the issue.  It doesn't appear to
affect other platforms, so it's inside a engine check
(`Util.Engine.trident === 6`).  Once we stop supporting
IE10, we should just remove this, because it's not the best
to have Engine-specific code.