Visual / layering fixes (#19547)

Package of a few visual changes to make things prettier-er. Also added a
helper for mapping purposes.

  - bugfix: "Carpets now show proper edges again."
- bugfix: "Fixed layering of several map items, like disposal pipes,
that were previously buried underneath turfs in the map editor (and were
invisible as a result)."
- bugfix: "Fixed observer sprites occasionally going underneath certain
objects. Their layering has been fixed."
- bugfix: "Reinforced plastic tables now show up correctly in the map
editor."
- rscadd: "Added a special 'lock' door helper that automatically locks
doors underneath it so mappers don't have to edit variables/icon states
manually."


![image](https://github.com/Aurorastation/Aurora.3/assets/55491249/53d97049-0104-491a-9a56-4d677afec536)

![image](https://github.com/Aurorastation/Aurora.3/assets/55491249/a26c9cf6-1ea9-4654-bc5c-954c878efda3)

![image](https://github.com/Aurorastation/Aurora.3/assets/55491249/6dbee17e-6a24-4cc9-b194-4c46530020be)

Fixes #18762 
Fixes #18840 
Fixes #19101

code/modules/effects/map_effects/door_helper.dm

@@ -59,3 +59,14 @@
 	"red" = list(19,38),
 	"delta" = list(19,38)
 )
+
+/// Locks/bolts any (lockable) door/airlock this marker is placed on.
+/obj/effect/map_effect/door_helper/lock
+	icon_state = "locked"
+
+/obj/effect/map_effect/door_helper/lock/modify_door(obj/machinery/door/D)
+	. = ..()
+	if(isairlock(D))
+		var/obj/machinery/door/airlock/A = D
+		A.locked = TRUE
+		A.set_airlock_overlays(AIRLOCK_CLOSED)