Gas canisters correctly update their appearance (#72124)

## About The Pull Request
Fixes issue #71712 & #72140 Don't know how the author wasn't able to
reproduce it even though it happens all the time on all maps


https://user-images.githubusercontent.com/110812394/208730887-7ad4b02d-9dcc-4d71-b54f-489c3bd616f6.mp4

## The Problem
The current code only updates the appearance of the canister when its
valve is opened & if it takes damage . It doesn't deal with the case
when its wrenched normally to a port without any problems

## The Solution
Update the appearance during processing but only when its pressure
changes significantly for the indicator light to be notified about it.
This way we don't have to care if
- the valve is open
- its wrenched to a port
- its leaking & taking damage

Regardless of what the cause is only if the pressure inside the canister
changes significantly from its current value do we update the
appearance.

This is so much more optimal than the previous code as`
update_appearance()` will be called a maximum of 8 times during its full
cycle of losing & refilling gas and won't be called at all if there is
no pressure change

## Changelog
🆑
fix: correctly update canister appearance
/🆑

Co-authored-by: LemonInTheDark <58055496+LemonInTheDark@users.noreply.github.com>

code/modules/atmospherics/machinery/portable/canister.dm

@@ -89,6 +89,9 @@ GLOBAL_LIST_INIT(gas_id_to_canister, init_gas_id_to_canister())
 
 	var/protected_contents = FALSE
 
+	///used while processing to update appearance only when its pressure state changes
+	var/current_pressure_state
+
 /obj/machinery/portable_atmospherics/canister/Initialize(mapload, datum/gas_mixture/existing_mixture)
 	. = ..()
 
@@ -379,21 +382,10 @@ GLOBAL_LIST_INIT(gas_id_to_canister, init_gas_id_to_canister())
 	if(connected_port)
 		. += mutable_appearance(canister_overlay_file, "can-connector")
 
-	var/air_pressure = air_contents.return_pressure()
-
-	switch(air_pressure)
-		if((40 * ONE_ATMOSPHERE) to INFINITY)
-			. += mutable_appearance(canister_overlay_file, "can-3")
-			. += emissive_appearance(canister_overlay_file, "can-3-light", src, alpha = src.alpha)
-		if((10 * ONE_ATMOSPHERE) to (40 * ONE_ATMOSPHERE))
-			. += mutable_appearance(canister_overlay_file, "can-2")
-			. += emissive_appearance(canister_overlay_file, "can-2-light", src, alpha = src.alpha)
-		if((5 * ONE_ATMOSPHERE) to (10 * ONE_ATMOSPHERE))
-			. += mutable_appearance(canister_overlay_file, "can-1")
-			. += emissive_appearance(canister_overlay_file, "can-1-light", src, alpha = src.alpha)
-		if((10) to (5 * ONE_ATMOSPHERE))
-			. += mutable_appearance(canister_overlay_file, "can-0")
-			. += emissive_appearance(canister_overlay_file, "can-0-light", src, alpha = src.alpha)
+	var/light_state = get_pressure_state(air_contents.return_pressure())
+	if(light_state) //happens when pressure is below 10kpa which means no light
+		. += mutable_appearance(canister_overlay_file, light_state)
+		. += emissive_appearance(canister_overlay_file, "[light_state]-light", src, alpha = src.alpha)
 
 	update_window()
 
@@ -572,6 +564,20 @@ GLOBAL_LIST_INIT(gas_id_to_canister, init_gas_id_to_canister())
 		else
 			shielding_powered = FALSE
 
+///return the icon_state component for the canister's indicator light based on its current pressure reading
+/obj/machinery/portable_atmospherics/canister/proc/get_pressure_state(air_pressure)
+	switch(air_pressure)
+		if((40 * ONE_ATMOSPHERE) to INFINITY)
+			return "can-3"
+		if((10 * ONE_ATMOSPHERE) to (40 * ONE_ATMOSPHERE))
+			return "can-2"
+		if((5 * ONE_ATMOSPHERE) to (10 * ONE_ATMOSPHERE))
+			return "can-1"
+		if((10) to (5 * ONE_ATMOSPHERE))
+			return "can-0"
+		else
+			return null
+
 /obj/machinery/portable_atmospherics/canister/process_atmos()
 	if(machine_stat & BROKEN)
 		return PROCESS_KILL
@@ -579,7 +585,6 @@ GLOBAL_LIST_INIT(gas_id_to_canister, init_gas_id_to_canister())
 		valve_open = !valve_open
 		timing = FALSE
 
-	var/visual_update = FALSE
 	// Handle gas transfer.
 	if(valve_open)
 		var/turf/location = get_turf(src)
@@ -587,17 +592,20 @@ GLOBAL_LIST_INIT(gas_id_to_canister, init_gas_id_to_canister())
 		excited = TRUE
 
 		if(air_contents.release_gas_to(target_air, release_pressure))
-			visual_update = TRUE
 			if(!holding)
 				air_update_turf(FALSE, FALSE)
 
 	// A bit different than other atmos devices. Wont stop if currently taking damage.
 	if(take_atmos_damage())
-		visual_update = TRUE
-		excited = TRUE
-
-	if(visual_update)
 		update_appearance()
+		excited = TRUE
+		return ..() //we have already updated appearance so dont need to update again below
+
+	var/new_pressure_state = get_pressure_state(air_contents.return_pressure())
+	if(current_pressure_state != new_pressure_state) //update apperance only when its pressure changes significantly from its current value
+		update_appearance()
+		current_pressure_state = new_pressure_state
+
 	return ..()
 
 /obj/machinery/portable_atmospherics/canister/ui_state(mob/user)