Refactor ExperimentalMapGenerator into reusable methods

This change lifts large portions of ExperimentalMapGenerator's logic
into a new "Terraformer" class with highly documented methods that can
theoretically be used by alternative map generator classes. Some
additional refactoring may occur in future, subject to the practical
needs of additional map generators. ClearMapGenerator is also
simplified.

This is not a pure refactor and contains some algorithmic and
behavioral changes, as well as few minor bug fixes. Notably:

- The logic for obstructing unreachable water has been somewhat
  replaced.

- In CnC, where water is already unplayable, EMG no longer obstructs
  water. (RA still does, as water is playable there.)

- Mountain (cliff) generation is now somewhat more effective. This
  increases the number of cliffs seen on many presets. Settings should
  no longer use "Mountains: 1000".

- PlayableSpace logic has been consolidated into Terraformer.

- PlayableSpace.Playability no longer exists as PartiallyPlayable
  became redundant. Its uses have been replaced with a simple
  boolean. Consequently, some defunct code and configuration has been
  removed.

- Adjust MultiBrush replaceability contract painting behavior to be
  more intuitive.

- Fix off-by-one error in map bounds computation.

- Fix some usages of mixed up MersenneTwisters.

OpenRA.Mods.Common/MapGenerator/ActorPlan.cs

@@ -145,5 +145,16 @@ namespace OpenRA.Mods.Common.MapGenerator
 
 			return CellLayerUtils.CVecToWVec(new CVec(left + right, top + bottom), Map.Grid.Type) / 2;
 		}
+
+		/// <summary>Return the larger of the width or height of the actor's footprint.</summary>
+		public int MaxSpan()
+		{
+			var footprint = Footprint().Select(kv => kv.Key).ToList();
+			var minX = footprint.Min(cpos => cpos.X);
+			var minY = footprint.Min(cpos => cpos.Y);
+			var maxX = footprint.Max(cpos => cpos.X);
+			var maxY = footprint.Max(cpos => cpos.Y);
+			return Math.Max(maxX - minX, maxY - minY) + 1;
+		}
 	}
 }