Fix actuator_velocity indexing in the LuGre bristle update.

mj_nextActivation indexed actuator_velocity with the actuator id, but the
array is laid out by output address. The two coincide only when all
preceding actuators are single-output, so a DC motor placed after a
multi-output SO3 actuator integrated its bristle state with another
actuator's velocity, while act_dot (computed in mj_fwdActuation with the
correct velocity) disagreed with the actual evolution of the state.
Fixes DCMotorTest.LuGreBristleVelocityOrderInvariance.

src/engine/engine_support.c

@@ -735,7 +735,7 @@ mjtNum mj_nextActivation(const mjModel* m, const mjData* d,
       mjtNum F_S = biasprm[4];    // static friction
       mjtNum v_S = biasprm[5];    // Stribeck velocity
       mjtNum sigma0 = dynprm[5];  // bristle stiffness
-      mjtNum velocity = d->actuator_velocity[actuator_id];
+      mjtNum velocity = d->actuator_velocity[m->actuator_outadr[actuator_id]];
       mjtNum g = mj_lugreStribeck(velocity, F_C, F_S, v_S);
 
       // ZOH exact ZOH integration: z(h) = exp(ah)*z(0) + ((exp(ah)-1)/a)*v