made some progress in saving and restoring the state during the simulation, with identical results.

Option to de/serialize btPersistentContactManifolds and fix lossy conversion during btMultiBody de/serialization of base world transform
(serialize the quaternion, not the converted 3x3 matrix)
There are still several caches not taken into account, and btMultiBody links/constraints are not deserialized yet etc.
See examples\pybullet\examples\saveRestoreState.py for an example.

src/BulletCollision/CollisionDispatch/btCollisionWorld.h

@@ -107,6 +107,9 @@ protected:
 
 	void	serializeCollisionObjects(btSerializer* serializer);
 
+	void serializeContactManifolds(btSerializer* serializer);
+
+
 public:
 
 	//this constructor doesn't own the dispatcher and paircache/broadphase