Optimizes lighting overlays for slow clients.

Ports https://github.com/tgstation/tgstation/pull/25778 and https://github.com/tgstation/tgstation/pull/25861
* Instead of using color matrix for all overlays, fully dark and fully light use static black and transparant icon states without color matrix.
* Also implements LIGHTING_ROUND_VALUE so that lighting values actually will reach 0 or 1.

code/modules/lighting/lighting_overlay.dm

@@ -59,16 +59,52 @@
 
 	var/max = max(cr.cache_mx, cg.cache_mx, cb.cache_mx, ca.cache_mx)
 
-	color  = list(
-		cr.cache_r, cr.cache_g, cr.cache_b, 0,
-		cg.cache_r, cg.cache_g, cg.cache_b, 0,
-		cb.cache_r, cb.cache_g, cb.cache_b, 0,
-		ca.cache_r, ca.cache_g, ca.cache_b, 0,
-		0, 0, 0, 1
+	var/rr = cr.cache_r
+	var/rg = cr.cache_g
+	var/rb = cr.cache_b
+
+	var/gr = cg.cache_r
+	var/gg = cg.cache_g
+	var/gb = cg.cache_b
+
+	var/br = cb.cache_r
+	var/bg = cb.cache_g
+	var/bb = cb.cache_b
+
+	var/ar = ca.cache_r
+	var/ag = ca.cache_g
+	var/ab = ca.cache_b
+
+	#if LIGHTING_SOFT_THRESHOLD != 0
+	var/set_luminosity = max > LIGHTING_SOFT_THRESHOLD
+	#else
+	// Because of floating points, it won't even be a flat 0.
+	// This number is mostly arbitrary.
+	var/set_luminosity = max > 1e-6
+	#endif
+
+	if((rr & gr & br & ar) && (rg + gg + bg + ag + rb + gb + bb + ab == 8))
+	//anything that passes the first case is very likely to pass the second, and addition is a little faster in this case
+		icon_state = "transparent"
+		color = null
+	else if(!set_luminosity)
+		icon_state = "dark"
+		color = null
+	else
+		icon_state = null
+		color = list(
+			rr, rg, rb, 00,
+			gr, gg, gb, 00,
+			br, bg, bb, 00,
+			ar, ag, ab, 00,
+			00, 00, 00, 01
 		)
-	luminosity = max > LIGHTING_SOFT_THRESHOLD
 
+	luminosity = set_luminosity
 
+// Variety of overrides so the overlays don't get affected by weird things.
+/atom/movable/lighting_overlay/ex_act()
+	return
 
 /atom/movable/lighting_overlay/singularity_act()
 	return