mech nitrogen filtering 2 (#19868)

* target oxygen logic

* filter out nitrogen when low on oxygen

* vvrw and datafield for everything

* :trollface:

* bruh does work

* tagless chicken

* move into atmos, make it not depend on mech

* update mech prototype

---------

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Server/Mech/Systems/MechSystem.cs

@@ -47,8 +47,6 @@ public sealed partial class MechSystem : SharedMechSystem
 
         _sawmill = Logger.GetSawmill("mech");
 
-        InitializeFiltering();
-
         SubscribeLocalEvent<MechComponent, InteractUsingEvent>(OnInteractUsing);
         SubscribeLocalEvent<MechComponent, EntInsertedIntoContainerMessage>(OnInsertBattery);
         SubscribeLocalEvent<MechComponent, MapInitEvent>(OnMapInit);
@@ -69,6 +67,8 @@ public sealed partial class MechSystem : SharedMechSystem
         SubscribeLocalEvent<MechPilotComponent, ExhaleLocationEvent>(OnExhale);
         SubscribeLocalEvent<MechPilotComponent, AtmosExposedGetAirEvent>(OnExpose);
 
+        SubscribeLocalEvent<MechAirComponent, GetFilterAirEvent>(OnGetFilterAir);
+
         #region Equipment UI message relays
         SubscribeLocalEvent<MechComponent, MechGrabberEjectMessage>(ReceiveEquipmentUiMesssages);
         SubscribeLocalEvent<MechComponent, MechSoundboardPlayMessage>(ReceiveEquipmentUiMesssages);
@@ -423,5 +423,17 @@ public sealed partial class MechSystem : SharedMechSystem
 
         args.Handled = true;
     }
+
+    private void OnGetFilterAir(EntityUid uid, MechAirComponent comp, ref GetFilterAirEvent args)
+    {
+        if (args.Air != null)
+            return;
+
+        // only airtight mechs get internal air
+        if (!TryComp<MechComponent>(uid, out var mech) || !mech.Airtight)
+            return;
+
+        args.Air = comp.Air;
+    }
     #endregion
 }