crate shjelf layer fix (#2597)

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## About The Pull Request
@thgvr 

![image](https://github.com/shiptest-ss13/Shiptest/assets/34109002/ae3c8e9f-bf0f-490a-851d-e7587c7fb1a8)
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## Why It's Good For The Game
fix bug
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## Changelog

:cl:
fix: crate shelves no longer have layering issues
/:cl:

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code/game/objects/structures/crateshelf.dm

@@ -24,7 +24,10 @@
 	var/stack_layer // This is used to generate the sprite layering of the shelf pieces.
 	var/stack_offset // This is used to generate the vertical offset of the shelf pieces.
 	for(var/i in 1 to (capacity - 1))
-		stack_layer  = BELOW_OBJ_LAYER + (0.02 * i) - 0.01 // Make each shelf piece render above the last, but below the crate that should be on it.
+		if(i >= 3) // If we're at or above three, we'll be on the way to going off the tile we're on. This allows mobs to be below the shelf when this happens.
+			stack_layer = ABOVE_MOB_LAYER + (0.02 * i) - 0.01
+		else
+			stack_layer  = BELOW_OBJ_LAYER + (0.02 * i) - 0.01 // Make each shelf piece render above the last, but below the crate that should be on it.
 		stack_offset = DEFAULT_SHELF_VERTICAL_OFFSET * i // Make each shelf piece physically above the last.
 		overlays += image(icon = 'icons/obj/objects.dmi', icon_state = "shelf_stack", layer = stack_layer, pixel_y = stack_offset)
 	return
@@ -85,7 +88,10 @@
 		shelf_contents[next_free] = crate // Insert a reference to the crate into the free slot.
 		crate.forceMove(src) // Insert the crate into the shelf.
 		crate.pixel_y = DEFAULT_SHELF_VERTICAL_OFFSET * (next_free - 1) // Adjust the vertical offset of the crate to look like it's on the shelf.
-		crate.layer = BELOW_OBJ_LAYER + 0.02 * (next_free - 1) // Adjust the layer of the crate to look like it's in the shelf.
+		if(next_free >= 3) // If we're at or above three, we'll be on the way to going off the tile we're on. This allows mobs to be below the crate when this happens.
+			crate.layer = ABOVE_MOB_LAYER + 0.02 * (next_free - 1)
+		else
+			crate.layer = BELOW_OBJ_LAYER + 0.02 * (next_free - 1) // Adjust the layer of the crate to look like it's in the shelf.
 		handle_visuals()
 		return TRUE
 	return FALSE // If the do_after() is interrupted, return FALSE!