Bullet bump up to version 2.88

add preliminary support to import btMultiBody from a .bullet file (will help save/restore state)
fix some Windows char/widechar issues

src/BulletDynamics/Featherstone/btMultiBodyDynamicsWorld.cpp

@@ -990,4 +990,17 @@ void	btMultiBodyDynamicsWorld::serializeMultiBodies(btSerializer* serializer)
 		}
 	}
 
+	//serialize all multibody links (collision objects)
+	for (i=0;i<m_collisionObjects.size();i++)
+	{
+		btCollisionObject* colObj = m_collisionObjects[i];
+		if (colObj->getInternalType() == btCollisionObject::CO_FEATHERSTONE_LINK)
+		{
+			int len = colObj->calculateSerializeBufferSize();
+			btChunk* chunk = serializer->allocate(len,1);
+			const char* structType = colObj->serialize(chunk->m_oldPtr, serializer);
+			serializer->finalizeChunk(chunk,structType,BT_MB_LINKCOLLIDER_CODE,colObj);
+		}
+	}
+
 }