Ensure the MatterJS Demo doesn't stop bouncing

[bryanbraun]

Currently, if you watch the MatterJS demo for long enough, it stops bouncing. Ideally it would keep bouncing forever like the other demos.

This is a known issue on MatterJS: https://github.com/liabru/matter-js/issues/256

That issue remains open but there may be some ideas that we can try in the recent comments (I already tried the many-sided polygon thing, which maybe helps a little, but the issue still remains).

I'm mostly creating this issue as a reminder to keep an eye on that issue. Once they have a recommended solution, we should adopt it here.