Fixes some Almayer Walls randomly changing icons when hit (#4936)

# About the pull request

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Horizontal Almayer walls have "decoration" variants that are placed
randomly. Because these are applied in update_icon randomly, if you
shoot such a wall, it will cycle randomly through available icons.

On top of that, because the replacement icon was applied as an overlay,
it was drawn over everything else masking bullet and damage overlays.

This fixes both issues by making it use the icon instead and rolling
decorations ahead of time.

# Explain why it's good for the game
Consistency. Awfully distracting if you're in the firing range too

# Changelog
:cl:
fix: Horizontal Almayer walls no longer change icon randomly, and now
properly display damage overlays.
/:cl:

code/game/turfs/walls/wall_types.dm

@@ -28,15 +28,23 @@
 		/obj/structure/machinery/cm_vending/sorted/cargo_guns/cargo/blend,
 	)
 
+	/// The type of wall decoration we use, to avoid the wall changing icon all the time
+	var/decoration_type
+
+/turf/closed/wall/almayer/Initialize(mapload, ...)
+	if(!special_icon && prob(20))
+		decoration_type = rand(0,3)
+	return ..()
+
 /turf/closed/wall/almayer/update_icon()
-	..()
-	if(special_icon)
-		return
+	if(decoration_type == null)
+		return ..()
 	if(neighbors_list in list(EAST|WEST))
-		var/r1 = rand(0,10) //Make a random chance for this to happen
-		var/r2 = rand(0,3) // Which wall if we do choose it
-		if(r1 >= 9)
-			overlays += image(icon, icon_state = "almayer_deco_wall[r2]")
+		special_icon = TRUE
+		icon_state = "almayer_deco_wall[decoration_type]"
+	else // Wall connection was broken, return to normality
+		special_icon = FALSE
+	return ..()
 
 /turf/closed/wall/almayer/take_damage(dam, mob/M)
 	var/damage_check = max(0, damage + dam)