Makes bone-gel bottles slightly transparent (#5675)

# About the pull request

Adds 4 new sprites for bone-gel depending on how full it is.

Base (full bottle) sprite is unchanged.

# Explain why it's good for the game

Seeing at a glance whether you will have enough bone-gel in your
bone-gel is better than reading examine text or finding out when you
arrive at your bone-gel surgery step and come up short.

# Testing Photographs and Procedure

<details>
<summary>Screenshots & Videos</summary>


![bone-gel_animated](https://github.com/cmss13-devs/cmss13/assets/8878624/06ea9f79-0ec1-43e2-a995-b33b2f1c15f1)

![image](https://github.com/cmss13-devs/cmss13/assets/8878624/8547d181-21a2-43c9-9452-ed7cb47e965f)

</details>

# Changelog

:cl:
imageadd: added 4 bone-gel fill level sprite states
/:cl:

---------

Co-authored-by: Megaddd <>
Co-authored-by: BeagleGaming1 <[email protected]>

code/game/objects/items/tools/surgery_tools.dm

@@ -201,6 +201,8 @@
 	icon_state = "bone-gel"
 	w_class = SIZE_SMALL
 	matter = list("plastic" = 7500)
+	///base icon state for update_icon() to reference, fixes bonegel/empty
+	var/base_icon_state = "bone-gel"
 	///percent of gel remaining in container
 	var/remaining_gel = 100
 	///If gel is used when doing bone surgery
@@ -215,6 +217,22 @@
 	///How much bone gel is needed to mend bones
 	var/mend_bones_fix_cost = 5
 
+/obj/item/tool/surgery/bonegel/update_icon()
+	. = ..()
+	if(remaining_gel >= 100)
+		icon_state = base_icon_state
+		return
+	if(remaining_gel > 50)
+		icon_state = "[base_icon_state]_75"
+		return
+	if(remaining_gel > 25)
+		icon_state = "[base_icon_state]_50"
+		return
+	if(remaining_gel > 0)
+		icon_state = "[base_icon_state]_25"
+		return
+	icon_state = "[base_icon_state]_0"
+
 /obj/item/tool/surgery/bonegel/get_examine_text(mob/user)
 	. = ..()
 	if(unlimited_gel) //Only show how much gel is left if it actually uses bone gel
@@ -244,6 +262,7 @@
 		if(!do_after(user, time_per_refill, INTERRUPT_ALL, BUSY_ICON_FRIENDLY, refilling_obj))
 			break
 		remaining_gel = clamp(remaining_gel + 10, 0, 100)
+		update_icon()
 		to_chat(user, SPAN_NOTICE("[refilling_obj] chimes, and displays \"[remaining_gel]% filled\"."))
 
 	refilling = FALSE
@@ -257,14 +276,17 @@
 	if(remaining_gel < gel_cost)
 		return FALSE
 	remaining_gel -= gel_cost
+	update_icon()
 	return TRUE
 
 /obj/item/tool/surgery/bonegel/empty
 	remaining_gel = 0
+	icon_state = "bone-gel_0"
 
 /obj/item/tool/surgery/bonegel/predatorbonegel
 	name = "gel gun"
 	desc = "Inside is a liquid that is similar in effect to bone gel, but requires much smaller quantities, allowing near infinite use from a single capsule."
+	base_icon_state = "predator_bone-gel"
 	icon_state = "predator_bone-gel"
 	unlimited_gel = TRUE