Restores "soft" darksight vision.

The previous client-side optimization made lighting overlays go fully
opaque when it was dark, making it so species darksight didn't let you see
anything.  This fixes that by making a dark_soft icon state that has
opacity matching the LIGHTING_SOFT_THRESHOLD.

code/modules/lighting/lighting_overlay.dm

@@ -88,7 +88,7 @@
 		icon_state = "transparent"
 		color = null
 	else if(!set_luminosity)
-		icon_state = "dark"
+		icon_state = LIGHTING_ICON_STATE_DARK
 		color = null
 	else
 		icon_state = null