Fixes and creates variants of the Opaque Gas Mask (#5954)

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## About The Pull Request

Finally splits the opaque gas mask into one without any gas being
filtered out, and one which is designed for nitrogen breathers.

## Why It's Good For The Game

People requested the opaque mask be given a no-filter version for a long
while but nobody even tried to do it. Also brings the filter list up to
date.

## Changelog

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:cl:
add: New variant of the Opaque Mask
fix: Fixes gas paths for the Filtered version
/:cl:

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code/modules/clothing/masks/gasmask.dm

@@ -140,11 +140,16 @@
 
 /obj/item/clothing/mask/gas/opaque
 	name = "Opaque Mask"
-	desc = "A face-covering mask with an opaque faceplate that can be connected to an air supply, often used by various alien races to filter out oxygen."
+	desc = "A face-covering mask with an opaque faceplate that can be connected to an air supply. Despite being stripped of all advanced technolgy, it still seems airtight."
 	icon_state = "opaque_mask"
 	inv_hide_flags = null
-	filtered_gases = list(/datum/gas/phoron, /datum/gas/nitrous_oxide, /datum/gas/oxygen)
 
+/obj/item/clothing/mask/gas/opaque_nitrogen
+	name = "Opaque Mask - Oxygen Filter"
+	desc = "A face-covering mask with an opaque faceplate that can be connected to an air supply, often used by various alien races to filter out oxygen."
+	icon_state = "opaque_mask"
+	inv_hide_flags = null
+	filtered_gases = list(GAS_ID_PHORON, GAS_ID_NITROUS_OXIDE, GAS_ID_OXYGEN)
 
 /obj/item/clothing/mask/gas/syndicate
 	name = "tactical mask"

code/modules/loadout/loadout_mask.dm

@@ -57,3 +57,7 @@
 /datum/loadout_entry/mask/opaque
 	name = "Opaque Mask"
 	path = /obj/item/clothing/mask/gas/opaque
+
+/datum/loadout_entry/mask/opaque_nitrogen
+	name = "Opaque Mask - Oxygen Filter"
+	path = /obj/item/clothing/mask/gas/opaque_nitrogen