Add CnC map generator support

Adds map generator support for CnC (all tilesets), largely on par with
RA.

Most tilesets do not have a complete set of beach templates and
therefore do not support terrain settings involving water. DESERT is the
only tileset supporting water. Unlike in RA, water is not playable space
in CnC (no naval units), so only terrain settings with small bodies of
water are available.

Most changes are configuration (tileset and map generator config), with
just a small number of code changes.

OpenRA.Mods.Common/Lint/CheckMultiBrushes.cs

@@ -32,9 +32,6 @@ namespace OpenRA.Mods.Common.Lint
 								// Includes validation of actor types and template IDs.
 								var multiBrush = new MultiBrush(map, info);
 
-								// Validates there is at least something in the MultiBrush.
-								multiBrush.Contract();
-
 								foreach (var (_, tile) in multiBrush.Tiles)
 									if (!templatedTerrainInfo.TryGetTerrainInfo(tile, out var _))
 										emitError($"Tileset {terrainInfoName} has invalid MultiBrush collection `{collectionName}`: tileset does not have tile {tile.Type},{tile.Index}");