- Converted as much as I could find about turf overlays to use add_overlay().
- This should be enough to stop BYOND from crashing, so we can eliminate the turf_overlay_holder hack.
- This also lets us remove the anti-corruption hacks from walls and open space.
- ZAS gas overlays can use priority overlays, so this also fixes the gas-goes-away-when-crowbarring-plating issue.
- Stuff like that