Add threading primitives.

PiperOrigin-RevId: 562649644
Change-Id: I9c35b270e4b3b50cc7eb5a152f24d71def8e2fcd

src/engine/engine_io.c

@@ -1114,6 +1114,8 @@ static mjData* _makeData(const mjModel* m) {
     }
   }
 
+  d->threadpool = 0;
+
   return d;
 }
 
@@ -1202,6 +1204,8 @@ mjData* mj_copyData(mjData* dest, const mjModel* m, const mjData* src) {
     }
   }
 
+  dest->threadpool = src->threadpool;
+
   return dest;
 }