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/Traits/World/ClearMapGenerator.cs

@@ -9,11 +9,9 @@
  */
 #endregion
 
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using OpenRA.Mods.Common.MapGenerator;
-using OpenRA.Mods.Common.Terrain;
 using OpenRA.Support;
 using OpenRA.Traits;
 
@@ -76,43 +74,21 @@ namespace OpenRA.Mods.Common.Traits
 			var random = new MersenneTwister();
 			var terrainInfo = modData.DefaultTerrainInfo[args.Tileset];
 
-			var map = new Map(modData, terrainInfo, args.Size);
-			var maxTerrainHeight = map.Grid.MaximumTerrainHeight;
-			var tl = new PPos(1, 1 + maxTerrainHeight);
-			var br = new PPos(args.Size.Width - 1, args.Size.Height + maxTerrainHeight - 1);
-			map.SetBounds(tl, br);
-
 			if (!Exts.TryParseUshortInvariant(args.Settings.NodeWithKey("Tile").Value.Value, out var tileType))
 				throw new YamlException("Illegal tile type");
 
-			var tile = new TerrainTile(tileType, 0);
-			if (!terrainInfo.TryGetTerrainInfo(tile, out var _))
+			if (!terrainInfo.TryGetTerrainInfo(new TerrainTile(tileType, 0), out var _))
 				throw new MapGenerationException("Illegal tile type");
 
-			// If the default terrain tile is part of a PickAny template, pick
-			// a random tile index. Otherwise, just use the default tile.
-			Func<TerrainTile> tilePicker;
-			if (map.Rules.TerrainInfo is ITemplatedTerrainInfo templatedTerrainInfo &&
-				templatedTerrainInfo.Templates.TryGetValue(tileType, out var template) &&
-				template.PickAny)
-			{
-				tilePicker = () => new TerrainTile(tileType, (byte)random.Next(0, template.TilesCount));
-			}
-			else
-			{
-				tilePicker = () => tile;
-			}
+			var map = new Map(modData, terrainInfo, args.Size);
+			var terraformer = new Terraformer(args, map, modData, [], Symmetry.Mirror.None, 1);
 
-			foreach (var cell in map.AllCells)
-			{
-				var mpos = cell.ToMPos(map);
-				map.Tiles[mpos] = tilePicker();
-				map.Resources[mpos] = new ResourceTile(0, 0);
-				map.Height[mpos] = 0;
-			}
+			terraformer.InitMap();
 
-			map.PlayerDefinitions = new MapPlayers(map.Rules, 0).ToMiniYaml();
-			map.ActorDefinitions = [];
+			foreach (var mpos in map.AllCells.MapCoords)
+				map.Tiles[mpos] = terraformer.PickTile(random, tileType);
+
+			terraformer.BakeMap();
 
 			return map;
 		}