Code effeciency project: Canister icon updates

Before:  If a canister is hooked up to a connector port it's icons were being deleted and recreated every tick.

After:  We store an update_flag that is a bitfield of the current icon and overlay states, and created a proc to check if it should change.  If not, we don't mess with icons.  Real simple.

Conflicts:
	code/game/machinery/atmoalter/canister.dm

code/game/machinery/atmoalter/canister.dm

@@ -18,16 +18,7 @@
 	use_power = 0
 	var/release_log = ""
 	var/busy = 0
-/*	m_amt=10*CC_PER_SHEET_METAL
-
-// Need to redefine /obj/item's recyk method
-/obj/machinery/portable_atmospherics/canister/recycle(var/obj/machinery/mineral/processing_unit/recycle/rec)
-	if (src.m_amt == 0 && src.g_amt == 0)
-		return 0
-	rec.iron += src.m_amt/CC_PER_SHEET_METAL
-	rec.glass += src.g_amt/CC_PER_SHEET_GLASS
-	return 1
-*/
+	var/update_flag = 0
 
 /obj/machinery/portable_atmospherics/canister/sleeping_agent
 	name = "Canister: \[N2O\]"
@@ -60,42 +51,65 @@
 	_color = "grey"
 	can_label = 0
 
-/*
- * return 0 (reached the proc code end)
- *        1 (canister is destroyed)
- */
-/obj/machinery/portable_atmospherics/canister/update_icon()
-	var/L[0]
-	overlays = L
-
-	if (destroyed)
-		icon_state = "[_color]-1"
-		return 1
-
-	icon_state = "[_color]"
-
-	L += "can-o3"
+/obj/machinery/portable_atmospherics/canister/proc/check_change()
+	var/old_flag = update_flag
+	update_flag = 0
+	if(holding)
+		update_flag |= 1
+	if(connected_port)
+		update_flag |= 2
 
 	var/tank_pressure = air_contents.return_pressure()
+	if(tank_pressure < 10)
+		update_flag |= 4
+	else if(tank_pressure < ONE_ATMOSPHERE)
+		update_flag |= 8
+	else if(tank_pressure < 15*ONE_ATMOSPHERE)
+		update_flag |= 16
+	else
+		update_flag |= 32
 
-	if (tank_pressure < 10)
-		L[L.len] = "can-o0"
-	else if (tank_pressure < ONE_ATMOSPHERE)
-		L[L.len] = "can-o1"
-	else if (tank_pressure < 15 * ONE_ATMOSPHERE)
-		L[L.len] = "can-o2"
+	if(update_flag == old_flag)
+		return 1
+	else
+		return 0
 
-	tank_pressure = null
+/obj/machinery/portable_atmospherics/canister/update_icon()
+/*
+update_flag
+1 = holding
+2 = connected_port
+4 = tank_pressure < 10
+8 = tank_pressure < ONE_ATMOS
+16 = tank_pressure < 15*ONE_ATMOS
+32 = tank_pressure go boom.
+*/
 
-	if (holding)
-		L += "can-open"
+	if (src.destroyed)
+		src.overlays = 0
+		src.icon_state = text("[]-1", src._color)
 
-	if (connected_port)
-		L += "can-connector"
+	if(icon_state != "[_color]")
+		icon_state = "[_color]"
 
-	overlays = L
-	L = null
-	return 0
+	if(check_change()) //Returns 1 if no change needed to icons.
+		return
+
+	src.overlays = 0
+
+	if(update_flag & 1)
+		overlays += "can-open"
+	if(update_flag & 2)
+		overlays += "can-connector"
+	if(update_flag & 4)
+		overlays += "can-o0"
+	if(update_flag & 8)
+		overlays += "can-o1"
+	else if(update_flag & 16)
+		overlays += "can-o2"
+	else if(update_flag & 32)
+		overlays += "can-o3"
+	return
 
 /obj/machinery/portable_atmospherics/canister/temperature_expose(datum/gas_mixture/air, exposed_temperature, exposed_volume)
 	if(exposed_temperature > temperature_resistance)