Throttle/brake now apply a direct, ground-only force to the chassis on
top of spinning the wheel, since friction-mediated propulsion alone
couldn't reach a satisfying top speed. isGrounded() checks the engine's
live collision pairs instead of tallying start/end events, which left a
counter permanently stuck above zero when crossing segment seams and
turned airborne throttle into a rocket thruster.
Kid.destroy() and GForceMonitor.destroy() no longer call
matter.world.removeConstraint()/off(): by the time a scene's 'shutdown'
handler runs, the Matter World plugin has already torn down and nulled
this.matter.world, throwing 'Cannot read properties of null'. Matter's
World#shutdown() clears all bodies/constraints/listeners itself anyway,
so both removals were redundant, not just unsafe.
"
Brian Fertig