Implements fuel fires

Implements liquid fuel fires using the liquid fuel cleanable decal objects.
Rewrites zburn to be more sane, replaces convoluted calculations with straightforward reaction limit.
Fixes issue with liquid fuel spreading caused by thee object being deleted.
Prevents zones from being repeatedly added to the active fire zones list, and processed repeatedly.

code/game/objects/effects/decals/Cleanable/fuel.dm

@@ -1,10 +1,10 @@
-obj/effect/decal/cleanable/liquid_fuel
+/obj/effect/decal/cleanable/liquid_fuel
 	//Liquid fuel is used for things that used to rely on volatile fuels or phoron being contained to a couple tiles.
 	icon = 'icons/effects/effects.dmi'
 	icon_state = "fuel"
 	layer = TURF_LAYER+0.2
 	anchored = 1
-	var/amount = 1 //Basically moles.
+	var/amount = 1
 
 	New(turf/newLoc,amt=1,nologs=0)
 		if(!nologs)
@@ -16,25 +16,34 @@ obj/effect/decal/cleanable/liquid_fuel
 		for(var/obj/effect/decal/cleanable/liquid_fuel/other in newLoc)
 			if(other != src)
 				other.amount += src.amount
+				
+				var/oldsrc = src
+				src = null
 				spawn other.Spread()
-				del src
+				del(oldsrc)
 
 		Spread()
 		. = ..()
 
-	proc/Spread()
+	proc/Spread(exclude=list())
 		//Allows liquid fuels to sometimes flow into other tiles.
-		if(amount < 5.0) return
+		if(amount < 15) return //lets suppose welder fuel is fairly thick and sticky. For something like water, 5 or less would be more appropriate.
 		var/turf/simulated/S = loc
 		if(!istype(S)) return
 		for(var/d in cardinal)
-			if(rand(25))
-				var/turf/simulated/target = get_step(src,d)
-				var/turf/simulated/origin = get_turf(src)
-				if(origin.CanPass(null, target, 0, 0) && target.CanPass(null, origin, 0, 0))
-					if(!locate(/obj/effect/decal/cleanable/liquid_fuel) in target)
-						new/obj/effect/decal/cleanable/liquid_fuel(target, amount*0.25,1)
-						amount *= 0.75
+			var/turf/simulated/target = get_step(src,d)
+			var/turf/simulated/origin = get_turf(src)
+			if(origin.CanPass(null, target, 0, 0) && target.CanPass(null, origin, 0, 0))
+				var/obj/effect/decal/cleanable/liquid_fuel/other_fuel = locate() in target
+				if(other_fuel)
+					other_fuel.amount += amount*0.25
+					if(!(other_fuel in exclude))
+						exclude += src
+						other_fuel.Spread(exclude)
+				else
+					new/obj/effect/decal/cleanable/liquid_fuel(target, amount*0.25,1)
+				amount *= 0.75
+						
 
 	flamethrower_fuel
 		icon_state = "mustard"

code/game/objects/items/weapons/tanks/tanks.dm

@@ -246,7 +246,7 @@
 
 /obj/item/weapon/tank/process()
 	//Allow for reactions
-	air_contents.react()
+	air_contents.react() //cooking up air tanks - add phoron and oxygen, then heat above PHORON_MINIMUM_BURN_TEMPERATURE
 	check_status()