Update ReadMassProperties to reflect rapier's mass properties

[Aceeri]

Also protects the internal MassProperties from direct modification.

Currently a relatively un-optimized version where I don't even try to guess when it would change.

I think I'd like to change this to update only if one of these has changed:

• ColliderMassProperties
• Collider::scale
• AdditionalMassProperties

[Aceeri]

Should be a bit more optimized now, only updates the ReadMassProperties based on the changes mentioned above.

[sebcrozet]

Thanks! I renamed MassModified to MassModifiedEvent.