Atmospherics Multilayer Pipes (#30972)

* 1

* rpd

* 3

* 4

* 5

* 6

* 7

* 8

* wew

* src. and underlays

* update

* woops

* misspelled params

* reviews

* fixes

* fixes

* better checks and layer crap

* Update RPD.dm

* Update pipe_construction.dm

* 3 layers, fixes

* woops i didn't update defines correctly

* dunc

* Fixes

* Fixes

* fixes

* fixes some more bullshit

* fixes

* fixes

* not needed

* makes attachment sprites prettier or uglier i dunno

* Unary flag

* sprite fix??

* fixes...

* sprite fixes

code/modules/mob/mob_defines.dm

@@ -143,3 +143,5 @@
 	var/list/can_ride_typecache = list()
 
 	var/list/mousemove_intercept_objects
+
+	var/ventcrawl_layer = PIPING_LAYER_DEFAULT