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/CellLayer.cs

@@ -63,12 +63,16 @@ namespace OpenRA
 
 			var u = (x - y) / 2;
 			var v = x + y;
+			if (!Bounds.Contains(u, v))
+				throw new IndexOutOfRangeException();
 			return v * Size.Width + u;
 		}
 
 		// Resolve an array index from map coordinates
 		int Index(MPos uv)
 		{
+			if (!Bounds.Contains(uv.U, uv.V))
+				throw new IndexOutOfRangeException();
 			return uv.V * Size.Width + uv.U;
 		}
 
@@ -145,6 +149,9 @@ namespace OpenRA
 		{
 			return uv.Clamp(new Rectangle(0, 0, Size.Width - 1, Size.Height - 1));
 		}
+
+		public CellRegion CellRegion =>
+			new(GridType, new MPos(0, 0), new MPos(Size.Width - 1, Size.Height - 1));
 	}
 
 	// Helper functions