Demo: Balls don't always collide with screen borders

[icanzilb]

It feels like this might be a SpriteKit issue of some sort but when I run the demo app on iOS10 iPhone 6s Simulator the balls sometimes just wonder off the screen.

Does anyone want to have a look at this and send a PR if they find a fix?

[icanzilb]

maybe @hollance 👆 ?

[hollance]

This may be a timing issue, maybe where the delta time becomes "too large" and the next animation step jumps past the thing it's supposed to collide with. Or not. It's been ages since I looked at this code.

There used to be collision problems with some older version of SpriteKit but I think that was in the beta and was fixed at some point.