Eris low walls & windows.

Still needs work blocked by better entity parenting, but oh well.

Content.Client/GameObjects/EntitySystems/WindowSystem.cs

@@ -0,0 +1,51 @@
+using System.Collections.Generic;
+using Content.Client.GameObjects.Components;
+using Content.Client.GameObjects.Components.IconSmoothing;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects;
+using Robust.Shared.GameObjects.Components.Transform;
+using Robust.Shared.GameObjects.Systems;
+using Robust.Shared.Interfaces.GameObjects;
+using Robust.Shared.Map;
+
+namespace Content.Client.GameObjects.EntitySystems
+{
+    [UsedImplicitly]
+    public sealed class WindowSystem : EntitySystem
+    {
+        private readonly Queue<IEntity> _dirtyEntities = new Queue<IEntity>();
+
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            SubscribeEvent<WindowSmoothDirtyEvent>(HandleDirtyEvent);
+        }
+
+        private void HandleDirtyEvent(object sender, WindowSmoothDirtyEvent ev)
+        {
+            if (sender is IEntity senderEnt && senderEnt.HasComponent<WindowComponent>())
+            {
+                _dirtyEntities.Enqueue(senderEnt);
+            }
+        }
+
+        public override void FrameUpdate(float frameTime)
+        {
+            base.FrameUpdate(frameTime);
+
+            // Performance: This could be spread over multiple updates, or made parallel.
+            while (_dirtyEntities.Count > 0)
+            {
+                _dirtyEntities.Dequeue().GetComponent<WindowComponent>().UpdateSprite();
+            }
+        }
+    }
+
+    /// <summary>
+    ///     Event raised by a <see cref="WindowComponent"/> when it needs to be recalculated.
+    /// </summary>
+    public sealed class WindowSmoothDirtyEvent : EntitySystemMessage
+    {
+    }
+}