Misc tweaks (#3625)

changes:

Fastboot now turns off AO.
Removed some unused code from atmospheric machinery init.
Removed global list of all lighting overlays as it was unused.
Lighting now tracks the total number of light sources in the world.
Lighting now tracks the percentage of lighting updates done via. instant updates.

code/modules/lighting/lighting_source.dm

@@ -48,6 +48,7 @@
 	var/skip_falloff = FALSE	// ONLY for use with sunlight, behavior is undefined if TRUE on regular sources.
 
 /datum/light_source/New(atom/owner, atom/top)
+	SSlighting.total_lighting_sources++
 	source_atom = owner // Set our new owner.
 
 	LAZYADD(source_atom.light_sources, src)
@@ -72,6 +73,7 @@
 // Kill ourselves.
 /datum/light_source/Destroy(force)
 	//L_PROF(source_atom, "source_destroy")
+	SSlighting.total_lighting_sources--
 
 	remove_lum()
 	if (source_atom)
@@ -96,6 +98,7 @@
 			SSlighting.light_queue += src;              \
 		}                                               \
 		else {                                          \
+			SSlighting.total_instant_updates += 1;      \
 			update_corners(TRUE);                       \
 			needs_update = LIGHTING_NO_UPDATE;          \
 		}                                               \