effects/overview: Avoid relaying text during animation

There is a window title beneat the stock WindowHeapDelegate. The label
was bound to the size of the window, but because this animates our label
resizes every frame too.

Given the label is only at full opacity when the window is filling the
ExpoCell, we can set the width of our label to that directly.

This looks less glitchy as we don't change wrapping during animation,
but also should have a noticable performance impact as we avoid
re-laying out multiple times.