Store waypoints at map load instead of iterating through the thinker list to find them

To clarify, this is only for MT_TUBEWAYPOINT, which is used by zoom tubes, rope hangs and PolyObjects. Boss waypoints are not affected. This improves the performance of the "move by waypoints" PolyObject action drastically: On 2.2.0 CEZ2, the share of the running time spent in T_PolyObjWaypoint decreases from 8.64% to just 0.05%.

srb2win-waypoints.exe