Use scenery-phet.SkyNode

[pixelzoom]

From https://github.com/phetsims/scenery-phet/issues/79#issue-44468162:

Currently, many sims with sky node exhibit the same bug as phetsims/fluid-pressure-and-flow#87 including BalancingAct.

While working on correcting this problem, I noticed that balancing-act is not using SkyNode. Should it be?

It currently has its own implementation, OutsideBackgroundNode.

[jbphet]

@pixelzoom said:

[balancing-act] currently has its own implementation, OutsideBackgroundNode.

OutsideBackgroundNode is, I believe, a common code node in scenery-phet that uses the aforementioned scenery-phet.SkyNode in conjunction with a scenery-phet.GroundNode to create a background with both ground and sky. So, I'm thinking the implementation in balancing-act is fine as is. My guess is that @pixelzoom didn't realize that OutsideBackgroundNode was a scenery-phet thing.

Right?

[pixelzoom]

Yeah, that must've been it. Sorry for not digging deeper. For context, my quick review was for an ancient scenery-phet issue that I finally transferred to phetsims/fluid-pressure-and-flow#335 (comment).

Closing.