Reorganise ResourceLayer implementation.

OpenRA.Editor/RenderUtils.cs

@@ -77,7 +77,7 @@ namespace OpenRA.Editor
 
 		public static ResourceTemplate RenderResourceType(ResourceTypeInfo info, string[] exts, Palette p)
 		{
-			var image = info.SpriteNames[0];
+			var image = info.EditorSprite;
 			using (var s = FileSystem.OpenWithExts(image, exts))
 			{
 				// TODO: Do this properly

OpenRA.Editor/ResourceTool.cs

@@ -27,7 +27,7 @@ namespace OpenRA.Editor
 				= new TileReference<byte, byte>
 				{
 					Type = (byte)resourceTemplate.Info.ResourceType,
-					Index = (byte)random.Next(resourceTemplate.Info.SpriteNames.Length)
+					Index = (byte)random.Next(resourceTemplate.Info.MaxDensity)
 				};
 
 			var ch = new int2(surface.GetBrushLocation().X / Surface.ChunkSize,