RIGIDBODY_OT_object_add uses obmat to initialize the rigid body state.
RIGIDBODY_OT_constraint_add doesn't currently use the obmat, but might do
in future for intializing the relative frames in constrained bodies, so added it as UNUSED param.
RIGIDBODY_OT_world_export is unregistered atm, but uses state to export to bullet format.

General remarks:

• BKE_rigidbody_validate_sim_world creates a dynamics world in the original scene. Eventually i guess dynamics should happen at runtime in the eval scene instead (independent physics timeline), so the depsgraph should manage creating & validating the dynamics world instead of doing this in the operator.

• Check uses of scene->rigidbody_world: this should be replaced entirely by using scene_eval. Assume that original scene has no rigidbody_world at all?