Fix biactive lens instability.

When lenses face one another and are each fed light to
become active, they now prefer the active state over the
shining state.

Interestingly, inline vs. angled configurations have different
numbers of stable configurations...

docs/issues.txt

@@ -9,9 +9,6 @@ ISSUES: Bugs, Cleanup and Refinements
  #    #  #       #    #     #    #       #       #   #   #    #
   ####   #        ####      #    ######  ######  #    #   ####
 
-- Active lenses facing each other go apeshit.
-	- They also totally bog the game down with lighting recalcs.
-
 - Optics should be blockable by things in item form.
 	- If sunlight_propagates on the node, and it's groups.visinv,
 	  then also check sunlight_propagates of the item def if the

mods/nc_optics/lens.lua

@@ -8,16 +8,21 @@ local modname = minetest.get_current_modname()
 local function lens_check(pos, node, check)
 	local face = nodecore.facedirs[node.param2]
 
-	if check(face.k) then
+	local backfed = check(face.k)
+	if backfed and node.name == modname .. ":lens" then
 		return modname .. ":lens_glow"
 	end
 
 	local fore = vector.add(pos, face.f)
+	local on
+	if face.f.y == 1 then
 		local ll = minetest.get_node_light(fore)
-	if not ll then return end
+		if ll then
 			local lt = 15
 			if node and node.name == modname .. ":lens_on" then lt = 14 end
-	local on = ll >= lt and face.f.y == 1
+			on = ll >= lt and face.f.y == 1
+		end
+	end
 	if not on then
 		local nnode = minetest.get_node(fore)
 		local def = minetest.registered_items[nnode.name] or {}
@@ -26,6 +31,11 @@ local function lens_check(pos, node, check)
 	if on then
 		return modname .. ":lens_on", {face.k}
 	end
+
+	if backfed then
+		return modname .. ":lens_glow"
+	end
+
 	return modname .. ":lens"
 end