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.

Extras/Serialize/BulletFileLoader/btBulletFile.cpp

@@ -176,6 +176,10 @@ void btBulletFile::parseData()
 					//	listID->push_back((bStructHandle*)id);
 				}
 
+				if (dataChunk.code == BT_CONTACTMANIFOLD_CODE)
+				{
+					m_contactManifolds.push_back((bStructHandle*)id);
+				}
 				if (dataChunk.code == BT_MULTIBODY_CODE)
 				{
 					m_multiBodies.push_back((bStructHandle*)id);