[Manual Port] manually ports icon forge from Virgo (#9731)

code/controllers/configuration/entries/resources.dm

@@ -8,4 +8,6 @@
 
 /datum/config_entry/flag/cache_assets
 
+/datum/config_entry/flag/smart_cache_assets
+
 /datum/config_entry/flag/save_spritesheets

code/controllers/subsystems/asset_loading.dm

@@ -7,6 +7,7 @@ SUBSYSTEM_DEF(asset_loading)
 	flags = SS_NO_INIT
 	runlevels = RUNLEVEL_LOBBY|RUNLEVELS_DEFAULT
 	var/list/datum/asset/generate_queue = list()
+	var/last_queue_len = 0
 
 /datum/controller/subsystem/asset_loading/fire(resumed)
 	while(length(generate_queue))
@@ -16,7 +17,12 @@ SUBSYSTEM_DEF(asset_loading)
 
 		if(MC_TICK_CHECK)
 			return
+		last_queue_len = length(generate_queue)
 		generate_queue.len--
+	// We just emptied the queue
+	if(last_queue_len && !length(generate_queue))
+		// Clean up cached icons, freeing memory.
+		rustg_iconforge_cleanup()
 
 /datum/controller/subsystem/asset_loading/proc/queue_asset(datum/asset/queue)
 #ifdef DO_NOT_DEFER_ASSETS