Replace a bunch of overlay calls (#3005)

changes:

Converts a bunch of manual overlay manipulation to SSoverlay calls.
Moved more cache lists to SSicon_cache.

code/modules/organs/blood.dm

@@ -296,8 +296,8 @@ proc/blood_splatter(var/target,var/datum/reagent/blood/source,var/large)
 		B = new decal_type(T)
 
 	var/obj/effect/decal/cleanable/blood/drip/drop = B
-	if(istype(drop) && drips && drips.len && !large)
-		drop.overlays |= drips
+	if(istype(drop) && LAZYLEN(drips) && !large)
+		drop.add_overlay(drips)
 		drop.drips |= drips
 
 	// If there's no data to copy, call it quits here.