decal system & crayons (#5183)

Co-authored-by: Paul <ritter.paul1+git@googlemail.com>

Content.Client/Decals/DecalOverlay.cs

@@ -0,0 +1,60 @@
+using System.Collections.Generic;
+using Content.Shared.Decals;
+using Robust.Client.Graphics;
+using Robust.Client.Utility;
+using Robust.Shared.Enums;
+using Robust.Shared.Map;
+using Robust.Shared.Maths;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Utility;
+using Robust.Shared.Maths;
+
+namespace Content.Client.Decals
+{
+    public class DecalOverlay : Overlay
+    {
+        private readonly DecalSystem _system;
+        private readonly IMapManager _mapManager;
+        private readonly IPrototypeManager _prototypeManager;
+
+        public override OverlaySpace Space => OverlaySpace.WorldSpaceBelowEntities;
+
+        public DecalOverlay(DecalSystem system, IMapManager mapManager, IPrototypeManager prototypeManager)
+        {
+            _system = system;
+            _mapManager = mapManager;
+            _prototypeManager = prototypeManager;
+        }
+
+        protected override void Draw(in OverlayDrawArgs args)
+        {
+            var handle = args.WorldHandle;
+
+            Dictionary<string, SpriteSpecifier> cachedTextures = new();
+
+            SpriteSpecifier GetSpriteSpecifier(string id)
+            {
+                if (cachedTextures.TryGetValue(id, out var spriteSpecifier))
+                    return spriteSpecifier;
+
+                spriteSpecifier = _prototypeManager.Index<DecalPrototype>(id).Sprite;
+                cachedTextures.Add(id, spriteSpecifier);
+                return spriteSpecifier;
+            }
+
+            foreach (var (gridId, zIndexDictionary) in _system.DecalRenderIndex)
+            {
+                var grid = _mapManager.GetGrid(gridId);
+                handle.SetTransform(grid.WorldMatrix);
+                foreach (var (_, decals) in zIndexDictionary)
+                {
+                    foreach (var (_, decal) in decals)
+                    {
+                        var spriteSpecifier = GetSpriteSpecifier(decal.Id);
+                        handle.DrawTexture(spriteSpecifier.Frame0(), decal.Coordinates, decal.Angle, decal.Color);
+                    }
+                }
+            }
+        }
+    }
+}