Add AppearsOnMapPreview

Allows preplaced actors to be rendered on map preview when saving the
map.

Also removes requirements for ResourceLayer in OpenRA.Game and moves it
to OpenRA.Mods.Common.

OpenRA.Mods.Common/Traits/AppearsOnMapPreview.cs

@@ -0,0 +1,59 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+using System.Linq;
+using OpenRA.Graphics;
+using OpenRA.Primitives;
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Traits
+{
+	[Desc("Render this actor when creating the minimap while saving the map.")]
+	public class AppearsOnMapPreviewInfo : TraitInfo<AppearsOnMapPreview>, IMapPreviewSignatureInfo, Requires<IOccupySpaceInfo>
+	{
+		[Desc("Use this color to render the actor, instead of owner player color.")]
+		public readonly Color Color = new Color();
+
+		[Desc("Use this terrain color to render the actor, instead of owner player color.",
+			"Overrides `Color` if both set.")]
+		public readonly string Terrain = null;
+
+		void IMapPreviewSignatureInfo.PopulateMapPreviewSignatureCells(Map map, ActorInfo ai, ActorReference s, List<Pair<MPos, Color>> destinationBuffer)
+		{
+			var tileSet = map.Rules.TileSet;
+
+			Color color;
+			if (!string.IsNullOrEmpty(Terrain))
+			{
+				color = tileSet[tileSet.GetTerrainIndex(Terrain)].Color;
+			}
+			else if (Color != new Color())
+			{
+				color = Color;
+			}
+			else
+			{
+				var owner = map.PlayerDefinitions.Where(p => s.InitDict.Get<OwnerInit>().PlayerName == p.Value.Nodes.First(k => k.Key == "Name").Value.Value).First();
+				var colorValue = owner.Value.Nodes.Where(n => n.Key == "Color");
+				var ownerColor = colorValue.Any() ? colorValue.First().Value.Value : "FFFFFF";
+				Color.TryParse(ownerColor, out color);
+			}
+
+			var ios = ai.TraitInfo<IOccupySpaceInfo>();
+			var cells = ios.OccupiedCells(ai, s.InitDict.Get<LocationInit>().Value(null));
+			foreach (var cell in cells)
+				destinationBuffer.Add(new Pair<MPos, Color>(cell.Key.ToMPos(map), color));
+		}
+	}
+
+	public class AppearsOnMapPreview { }
+}