Malleable glass fix (#2564)

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## About The Pull Request

The "fill_icon" variable didn't seem to actually replace the icon file
for glasses


![image](https://github.com/shiptest-ss13/Shiptest/assets/24857008/0c786c51-c273-480f-8562-5cf0ebb6c031)

## Why It's Good For The Game

fixes #2372

## Changelog

🆑 
fix: malleable glasses now show their reagents again
/🆑

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code/modules/reagents/reagent_containers.dm

@@ -268,7 +268,7 @@
 		return
 
 	var/fill_name = fill_icon_state? fill_icon_state : icon_state
-	var/mutable_appearance/filling = mutable_appearance('icons/obj/reagentfillings.dmi', "[fill_name][fill_icon_thresholds[1]]")
+	var/mutable_appearance/filling = mutable_appearance(fill_icon, "[fill_name][fill_icon_thresholds[1]]")
 
 	var/percent = round((reagents.total_volume / volume) * 100)
 	for(var/i in 1 to fill_icon_thresholds.len)