Opacity refactor (#52881)

Moves all opacity var manipulation to a proc which sends a signal.
    light_blocker element for movable opaque atoms made, which tracks its movement and updates the affected turfs for proper lighting updates.
    has_opaque_atom boolean replaced by the opacity_sources lazylist to keep track of the sources, and a directional_opacity which serves a similar function but also allows for future expansion with on-border opaque objects (not yet implemented).
    Some opacity-related sight procs optimized as a result of this.
    Some variables moved to the object's definition.
    A define or two added into the mix for clarity.
    Some code cleaning, like turning booleans into their defines.
    One file renamed for clarity.

Changelog

cl
balance: Mechs no longer block sight. It's a non-trivial cost for the lighting system with little to no gain.
/cl

code/game/objects/structures/guncase.dm

@@ -6,7 +6,7 @@
 	icon_state = "shotguncase"
 	anchored = FALSE
 	density = TRUE
-	opacity = 0
+	opacity = FALSE
 	var/case_type = ""
 	var/gun_category = /obj/item/gun
 	var/open = TRUE