MacWolf pushwalls should become invisible at final destination

[Braden Obrzut]

In the Mac version of Wolf3D, pushwalls are only visible in their initial state and during movement. At the final destination (2 spaces away) they turn invisible. Typically this would be where a wall is baked into the map, but if it's an open space then it should turn into an invisible wall.

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This invisible wall should have the property of blocking AI targeting, but the player can still shoot through.