Minor refactors/fixes (#4589)

changes:

Underwear has been promoted to a human layer; this might fix some potential issues with human icon caching.
turf/Entered() code has been cleaned up and proximity checks have been timerized.
Fixed an issue where openspaces wouldn't render objects that entered an openspace after init.
Shortened organ keymap prefix as it didn't really need to be that long.

code/controllers/subsystems/icon_cache.dm

@@ -50,9 +50,6 @@
 	//    haironly:   nobeard_[hair_style][r_hair][g_hair][b_hair]
 	//    beardonly:  [beard_style][r_facial][g_facial][b_facial]_nohair
 	var/list/human_hair_cache = list()
-	var/list/human_underwear_cache = list()
-	var/list/human_undershirt_cache = list()
-	var/list/human_socks_cache = list()
 	var/list/organ_keymap = list()
 	var/current_organ_keymap_idex = 1
 	// This is an assoc list of all icon states in `icons/mob/collar.dmi`, used by human update-icons.
@@ -73,6 +70,8 @@
 	var/list/space_cache = list()
 	var/list/crayon_cache = list()
 
+	var/list/istate_cache = list()
+
 /datum/controller/subsystem/icon_cache/New()
 	NEW_SS_GLOBAL(SSicon_cache)
 
@@ -95,7 +94,7 @@
 	var/key = organ.get_mob_cache_key(FALSE)
 	. = organ_keymap[key]
 	if (!.)
-		organ_keymap[key] = "organ[current_organ_keymap_idex++]"
+		organ_keymap[key] = "o[current_organ_keymap_idex++]"
 		. = organ_keymap[key]
 
 /datum/controller/subsystem/icon_cache/proc/setup_uniform_mappings()
@@ -124,3 +123,35 @@
 		I.icon_state = istr
 		I.overlays += space_dust_cache[istr]
 		space_cache[istr] = I
+
+/datum/controller/subsystem/icon_cache/proc/get_state(icon/I, state)	// returns an APPEARANCE, not an image!
+	if (!isicon(I))
+		CRASH("Expected icon.")
+
+	if (!istext(state))
+		// non-fatal, so just print a message then carry on.
+		crash_with("Received non-text icon_state '[state || "(NULL)"]'; normalizing, but this shouldn't happen.")
+		state = "[state]"
+
+	var/list/cache = istate_cache[I]
+	if (cache)
+		if (cache[state])
+			return cache[state]
+	else
+		cache = preload_icon(I)
+
+	var/image/im = image(icon = I, icon_state = state)
+	return cache[state] = im.appearance
+
+// Loads all icon states in an icon into the istate cache.
+/datum/controller/subsystem/icon_cache/proc/preload_icon(icon/I)
+	log_debug("SSicon_cache: preloading '[I]'...")
+	var/list/cache = list()
+	var/image/im = new(icon = I)
+	for (var/state in icon_states(I))
+		im.icon_state = state
+		cache[state] = im.appearance
+
+	log_debug("SSicon_cache: preloaded [cache.len] states.")
+	istate_cache[I] = cache
+	return cache