Cooked mortar shell explosion is now fixed (#6638)

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# About the pull request
Fixed as in static, doesn't change for different mortar shell types
Makes the explosion when setting mortar shells on fire fixed and small
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# Explain why it's good for the game
This was supposed to be a niche feature like ammo boxes exploding and
instead it is being used to blow up inside pylons \ caves or even on
shipside. This is why we can't have nice things.
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# Changelog

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:cl:
del: Mortar shells no longer blow up their payload, instead they will
create a small fixed explosion.
/:cl:

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code/modules/cm_marines/equipment/mortar/mortar_shells.dm

@@ -167,23 +167,43 @@
 	return ..()
 
 /obj/item/mortar_shell/flamer_fire_act(dam, datum/cause_data/flame_cause_data)
+	addtimer(VARSET_CALLBACK(src, burning, FALSE), 5 SECONDS, TIMER_UNIQUE|TIMER_OVERRIDE|TIMER_DELETE_ME)
+
 	if(burning)
 		return
 	burning = TRUE
 	cause_data = create_cause_data("Burning Mortar Shell", flame_cause_data.resolve_mob(), src)
-	handle_fire()
+	handle_fire(cause_data)
 
-/obj/item/mortar_shell/proc/handle_fire()
-	visible_message(SPAN_WARNING("[src] catches on fire and starts cooking off! It's gonna blow!"))
-	anchored = TRUE // don't want other explosions launching it elsewhere
+/obj/item/mortar_shell/proc/can_explode()
+	return TRUE
 
-	var/datum/effect_system/spark_spread/sparks = new()
-	sparks.set_up(n = 10, loca = loc)
-	sparks.start()
-	new /obj/effect/warning/explosive(loc, 5 SECONDS)
+/obj/item/mortar_shell/custom/can_explode()
+	for(var/obj/item/reagent_container/glass/container in warhead?.containers)
+		for(var/datum/reagent/reagent in container?.reagents?.reagent_list)
+			if(reagent.explosive)
+				return TRUE
 
-	addtimer(CALLBACK(src, PROC_REF(detonate), loc), 5 SECONDS)
-	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(qdel), (src)), 5.5 SECONDS)
+	return FALSE
+
+/obj/item/mortar_shell/flare/can_explode()
+	return FALSE
+
+/obj/item/mortar_shell/proc/handle_fire(cause_data)
+	if(can_explode())
+		visible_message(SPAN_WARNING("[src] catches on fire and starts cooking off! It's gonna blow!"))
+		anchored = TRUE // don't want other explosions launching it elsewhere
+		var/datum/effect_system/spark_spread/sparks = new()
+		sparks.set_up(n = 10, loca = loc)
+		sparks.start()
+		new /obj/effect/warning/explosive(loc, 5 SECONDS)
+
+		addtimer(CALLBACK(src, PROC_REF(explode), cause_data), 5 SECONDS)
+		addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(qdel), (src)), 5.5 SECONDS)
+
+
+/obj/item/mortar_shell/proc/explode(flame_cause_data)
+	cell_explosion(src, 100, 25, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, flame_cause_data)
 
 /obj/structure/closet/crate/secure/mortar_ammo
 	name = "\improper M402 mortar ammo crate"