support for the interior tileSet. trees don't work. mounted all 3 of the tileSet mixen; this means there are name-clashes in some cases.

OpenRa.FileFormats/Walkability.cs

@@ -6,7 +6,7 @@ namespace OpenRa.FileFormats
 {
 	public class TileTemplate
 	{
-		public int Index;
+		public int Index; // not valid for `interior` stuff. only used for bridges.
 		public string Name;
 		public int2 Size;
 		public string Bridge;
@@ -36,10 +36,11 @@ namespace OpenRa.FileFormats
 							p => int.Parse(p.Key.Substring(8)),
 							p => int.Parse(p.Value)),
 					Name = section.GetValue("Name", null).ToLowerInvariant(),
-					Index = int.Parse(section.Name.Substring(3)),
 					Bridge = section.GetValue("bridge", null),
 					HP = float.Parse(section.GetValue("hp", "0"))
 				};
+				tile.Index = -1;
+				int.TryParse(section.Name.Substring(3), out tile.Index);
 
 				walkability[tile.Name] = tile;
 			}