Ported the core of the overlays management subsystem from /tg

- Added SSoverlays subsystem for compiling overlay lists and applying them to atoms in a controlled anti-lag subsystem.
- Added vars and procs to atom which should eventually replace all direct interaction with BYOND's /atom/overlays var outside the subsystem.
- Added OVERLAY_QUEUED flag to var/atom/flags bitfield.
- Added small framework for subsystem performance tracking. So far used only by SSoverlays
- Added admin debug command "Display overlay Log" to see performance stats mid-round.

code/modules/admin/admin_verbs.dm

@@ -195,6 +195,7 @@ var/list/admin_verbs_debug = list(
 	/client/proc/cmd_debug_tog_aliens,
 	/client/proc/cmd_display_del_log,
 	/client/proc/cmd_display_init_log,
+	/client/proc/cmd_display_overlay_log,
 	/client/proc/air_report,
 	/client/proc/reload_admins,
 	/client/proc/reload_eventMs,

code/modules/admin/verbs/debug.dm

@@ -335,6 +335,28 @@
 	if(!check_rights(R_DEBUG))	return
 	src << browse(replacetext(SSatoms.InitLog(), "\n", "<br>"), "window=initlog")
 
+/client/proc/cmd_display_overlay_log()
+	set category = "Debug"
+	set name = "Display overlay Log"
+	set desc = "Display SSoverlays log of everything that's passed through it."
+
+	if(!check_rights(R_DEBUG))	return
+	render_stats(SSoverlays.stats, src)
+
+// Render stats list for round-end statistics.
+/proc/render_stats(list/stats, user, sort = /proc/cmp_generic_stat_item_time)
+	sortTim(stats, sort, TRUE)
+
+	var/list/lines = list()
+	for (var/entry in stats)
+		var/list/data = stats[entry]
+		lines += "[entry] => [num2text(data[STAT_ENTRY_TIME], 10)]ms ([data[STAT_ENTRY_COUNT]]) (avg:[num2text(data[STAT_ENTRY_TIME]/(data[STAT_ENTRY_COUNT] || 1), 99)])"
+
+	if (user)
+		user << browse("<ol><li>[lines.Join("</li><li>")]</li></ol>", "window=[url_encode("stats:\ref[stats]")]")
+	else
+		. = lines.Join("\n")
+
 /client/proc/cmd_admin_grantfullaccess(var/mob/M in mob_list)
 	set category = "Admin"
 	set name = "Grant Full Access"