Cache light fixture overlays (#3948)

changes:

Light fixture overlays are now cached.
BlendRGB is now cached (used by light fixture overlays)
Nightmode now only tints the bulb instead of both the bulb and the fixture itself.

code/controllers/subsystems/icon_cache.dm

@@ -63,6 +63,10 @@
 
 	var/list/ao_cache = list()
 
+	var/list/light_fixture_cache = list()
+
+	var/list/rgb_blend_cache = list()	// not an icon per-se, but this proc could be expensive so we might as well cache it.
+
 /datum/controller/subsystem/icon_cache/New()
 	NEW_SS_GLOBAL(SSicon_cache)
 
@@ -85,3 +89,8 @@
 	uniform_states = list()
 	for (var/i in icon_states('icons/mob/uniform.dmi'))
 		uniform_states[i] = TRUE
+
+/datum/controller/subsystem/icon_cache/proc/generate_color_variant(icon/icon, icon_state, color)
+	var/image/I = new(icon, icon_state)
+	I.color = color
+	return I