Add experimental RA procedural map generator

Add an experimental procedural map generator for the Red Alert mod,
along with supporting code that may assist in the development of map
generators for other mods.

Map generation may be accessed as a tool in the Map Editor. This change
does not presently introduce direct lobby options for generated maps.

Features:

- Terrain with land, water, beaches, cliffs, roads, debris, and trees.
- Placement of mpspawns, neutral buildings, and resources.
- Rotational and mirror symmetry options.
- Various configurable parameters with presets.
- Deterministic with configurable seed.
- Performant.

OpenRA.Game/Map/TileReference.cs

@@ -25,6 +25,21 @@ namespace OpenRA
 		public override int GetHashCode() { return Type.GetHashCode() ^ Index.GetHashCode(); }
 
 		public override string ToString() { return Type + "," + Index; }
+
+		public static bool TryParse(string s, out TerrainTile tt)
+		{
+			var split = s.Split(',');
+			if (split.Length == 2 &&
+				Exts.TryParseUshortInvariant(split[0], out var type) &&
+				Exts.TryParseByteInvariant(split[1], out var index))
+			{
+				tt = new TerrainTile(type, index);
+				return true;
+			}
+
+			tt = default;
+			return false;
+		}
 	}
 
 	public readonly struct ResourceTile