fix: ensure the matterjs bounce speed is consistent

[bryanbraun]

Fixes #113.

Testing instructions:

1. Pull down this branch:
   • It's easy with gh: gh pr checkout 114
2. Install dependencies: npm ci && npm run build
3. Start the app: npm run start
4. In firefox, browse to http://localhost:3000/#matter-js and confirm that the ball bounces at the same speed as the other demos. Note: it's ok if the ball loses bounce height over time... that's captured in a different issue.