Screenshot tests (#67679) (#14269)

Adds screenshot visual testing workflow and scripts.

Co-authored-by: Mothblocks <35135081+Mothblocks@users.noreply.github.com>

code/modules/client/preferences/middleware/antags.dm

@@ -110,6 +110,9 @@
 	early = TRUE
 	cross_round_cachable = TRUE
 
+	/// Mapping of spritesheet keys -> icons
+	var/list/antag_icons = list()
+
 /datum/asset/spritesheet/antagonists/create_spritesheets()
 	// Antagonists that don't have a dynamic ruleset, but do have a preference
 	var/static/list/non_ruleset_antagonists = list(
@@ -129,7 +132,6 @@
 		antagonists[initial(ruleset.antag_flag)] = antagonist_type
 
 	var/list/generated_icons = list()
-	var/list/to_insert = list()
 
 	for (var/antag_flag in antagonists)
 		var/datum/antagonist/antagonist_type = antagonists[antag_flag]
@@ -138,7 +140,7 @@
 		var/spritesheet_key = serialize_antag_name(antag_flag)
 
 		if (!isnull(generated_icons[antagonist_type]))
-			to_insert[spritesheet_key] = generated_icons[antagonist_type]
+			antag_icons[spritesheet_key] = generated_icons[antagonist_type]
 			continue
 
 		var/datum/antagonist/antagonist = new antagonist_type
@@ -153,10 +155,10 @@
 		// If an icon is not prepared to be scaled to that size, it looks really ugly, and this
 		// makes it harder to figure out what size it *actually* is.
 		generated_icons[antagonist_type] = preview_icon
-		to_insert[spritesheet_key] = preview_icon
+		antag_icons[spritesheet_key] = preview_icon
 
-	for (var/spritesheet_key in to_insert)
-		Insert(spritesheet_key, to_insert[spritesheet_key])
+	for (var/spritesheet_key in antag_icons)
+		Insert(spritesheet_key, antag_icons[spritesheet_key])
 
 /// Serializes an antag name to be used for preferences UI
 /proc/serialize_antag_name(antag_name)