Ice anomaly spawns ice underneath it (#21227)

* added TileAnomalySystem to AnomalyIce

* added FloorIce for station

* created ice crust entity to spawn under ice anomaly

* update draw depth for ice crust

* uh oh, added ice-sliding but at what cost

* resolved mispredicts

* updated sprite alpha, removed appearance component (not used)

* fixed function not reflecting event name, left datafield attributes blank, added one comment about saving data (?)

---------

Co-authored-by: Yurii Kis <yurii.kis@smartteksas.com>

Content.Shared/Movement/Systems/FrictionContactsSystem.cs

@@ -0,0 +1,100 @@
+using Content.Shared.Movement.Components;
+using Robust.Shared.Physics.Components;
+using Robust.Shared.Physics.Events;
+using Robust.Shared.Physics.Systems;
+
+namespace Content.Shared.Movement.Systems;
+
+public sealed class FrictionContactsSystem : EntitySystem
+{
+    [Dependency] private readonly SharedPhysicsSystem _physics = default!;
+    [Dependency] private readonly MovementSpeedModifierSystem _speedModifierSystem = default!;
+
+    // Comment copied from "original" SlowContactsSystem.cs
+    // TODO full-game-save 
+    // Either these need to be processed before a map is saved, or slowed/slowing entities need to update on init.
+    private HashSet<EntityUid> _toUpdate = new();
+
+    public override void Initialize()
+    {
+        base.Initialize();
+        SubscribeLocalEvent<FrictionContactsComponent, StartCollideEvent>(OnEntityEnter);
+        SubscribeLocalEvent<FrictionContactsComponent, EndCollideEvent>(OnEntityExit);
+        SubscribeLocalEvent<FrictionContactsComponent, ComponentShutdown>(OnShutdown);
+
+        UpdatesAfter.Add(typeof(SharedPhysicsSystem));
+    }
+
+    private void OnEntityEnter(EntityUid uid, FrictionContactsComponent component, ref StartCollideEvent args)
+    {
+        var otherUid = args.OtherEntity;
+
+        if (!HasComp(otherUid, typeof(MovementSpeedModifierComponent)))
+            return;
+
+        _toUpdate.Add(otherUid);
+    }
+
+    private void OnEntityExit(EntityUid uid, FrictionContactsComponent component, ref EndCollideEvent args)
+    {
+        var otherUid = args.OtherEntity;
+
+        if (!HasComp(otherUid, typeof(MovementSpeedModifierComponent)))
+            return;
+
+        _toUpdate.Add(otherUid);
+    }
+
+    private void OnShutdown(EntityUid uid, FrictionContactsComponent component, ComponentShutdown args)
+    {
+        if (!TryComp(uid, out PhysicsComponent? phys))
+            return;
+
+        _toUpdate.UnionWith(_physics.GetContactingEntities(uid, phys));
+    }
+
+    public override void Update(float frameTime)
+    {
+        base.Update(frameTime);
+
+        foreach (var uid in _toUpdate)
+        {
+            ApplyFrictionChange(uid);
+        }
+
+        _toUpdate.Clear();
+    }
+
+    private void ApplyFrictionChange(EntityUid uid)
+    {
+        if (!EntityManager.TryGetComponent<PhysicsComponent>(uid, out var physicsComponent))
+            return;
+
+        if (!TryComp(uid, out MovementSpeedModifierComponent? speedModifier))
+            return;
+
+        FrictionContactsComponent? frictionComponent = TouchesFrictionContactsComponent(uid, physicsComponent);
+
+        if (frictionComponent == null)
+        {
+            _speedModifierSystem.ChangeFriction(uid, MovementSpeedModifierComponent.DefaultFriction, null, MovementSpeedModifierComponent.DefaultAcceleration, speedModifier);
+        }
+        else
+        {
+            _speedModifierSystem.ChangeFriction(uid, frictionComponent.MobFriction, frictionComponent.MobFrictionNoInput, frictionComponent.MobAcceleration, speedModifier);
+        }
+    }
+
+    private FrictionContactsComponent? TouchesFrictionContactsComponent(EntityUid uid, PhysicsComponent physicsComponent)
+    {
+        foreach (var ent in _physics.GetContactingEntities(uid, physicsComponent))
+        {
+            if (!TryComp(ent, out FrictionContactsComponent? frictionContacts))
+                continue;
+
+            return frictionContacts;
+        }
+
+        return null;
+    }
+}