Refactors how overlays handle the compile step (#38002)

Robustin says:

The very heart of our overlay code, a single line that basically boiled
down to overlays = new_overlays, was the cause of so much overlay lag.
Human overlay code was by far the biggest culprit. Most objects have 0-2
overlays but humans are marching around with 20+ most of the time and
the current system was spending a LOT of effort comparing 20+ image with
20+ other images and then apparently rendering them all anyway. Human
overlays are at least 10x the cost of any other overlay process and on a
busy server the overlay compiling was 2x the cost of any other system.

I compared the cost of overlay changes by picking up/dropping a PDA in
the dorms 250 times, with a 50% chance to use our current overlay
compiler and a 50% chance to use a "direct addition/removal (+=, -=)
approach:
CURRENT         1120ms  133     (avg:8.4210529327392578)
SCRAPS  6ms     112     (avg:0.0535714291036129)

Now this PR makes our whole overlay subsystem use that approach for
overlay compiling and the early results look incredible. The best part
is this is just the START of improvements. Humans benefits the most
because their icon system was already designed for small, incremental
overlay updates. By moving other code from "Cut everything, then put it
all back" to only updating the necessary overlays (cough, APC's), we can
see similar improvements.

oranges says:
I've picked up this PR because robustin vanished, but I do see the value
in the approach, only things I added were the checks for the overlay
list len changing

I also retabbed the entire file because I am a brainlet and did it
without thinking

code/__DEFINES/subsystems.dm

@@ -124,21 +124,19 @@
 
 #define COMPILE_OVERLAYS(A)\
 	if (TRUE) {\
-		var/list/oo = A.our_overlays;\
+		var/list/ad = A.add_overlays;\
+		var/list/rm = A.remove_overlays;\
 		var/list/po = A.priority_overlays;\
+		if(LAZYLEN(rm)){\
+			A.overlays -= rm;\
+			rm.Cut();\
+		}\
+		if(LAZYLEN(ad)){\
+			A.overlays |= ad;\
+			ad.Cut();\
+		}\
 		if(LAZYLEN(po)){\
-			if(LAZYLEN(oo)){\
-				A.overlays = oo + po;\
-			}\
-			else{\
-				A.overlays = po;\
-			}\
-		}\
-		else if(LAZYLEN(oo)){\
-			A.overlays = oo;\
-		}\
-		else{\
-			A.overlays.Cut();\
+			A.overlays |= po;\
 		}\
 		A.flags_1 &= ~OVERLAY_QUEUED_1;\
 	}