A physics world cache is a cache inferred from the contents of all rigid body components, fixtures components
and all types of joint components (e.g. motor joint component).
In practice, it means that it stores the physics world object (e.g. Box2D’s b2World) and pointers to its native body, fixture, shape and joint types.

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Copy-assignment operator

Unfortunately, Box2D does not provide any copy-assignment operator for b2World.
Such a functionality is be essential during client-side prediction of server state.

We currently have a rudimentary operator= that appears to work, but PlayRho appears to have its own that at least looks nice.
We might at some point switch to this library because of the fixed-size calculations.

We keep the significant state in our own format which should enable us to transition all potentially existing content rather smoothly.