Overlays with open={true} by default don't render

[etr2460]

I believe the change made in #287 resulted in Overlays not rendering if the open param is set to true initially. This seems to be because the Overlay doesn't render its children until it updates once, and if we initialize open to true then it never updates.

You should be able to repro this with a simple case like:

<Overlay open={true} onClose={() => undefined}>
  <div>This doesn't display</div>
</Overlay>

[williaster]

@alecklandgraf should this line in your PR be

if (x !== this.state.x || y !== this.state.y || !this.state.targetRectReady)