Fix CnC generated map oceans becoming debris-filled

Water is not playable in CnC, so, the map generator may fill it in if
it's not attributed to the largest playable region. This would often
happen for circular-in-water maps when DenyWalledAreas is enabled.

This change adds a step to adopt any neighboring partially playable
space (such as beaches and water) into the largest region, which is
exempt from debris filling.

The change is almost purely cosmetic, though the previous debris filling
could slightly reduce playable space (due to filling playable beach
tiles), possibly causing generation to fail. As such, map generation is
now a very tiny bit more reliable.

OpenRA.Mods.Common/Traits/World/ExperimentalMapGenerator.cs

@@ -1013,6 +1013,26 @@ namespace OpenRA.Mods.Common.Traits
 				if (largest.PlayableArea < minimumPlayableSpace)
 					throw new MapGenerationException("playable space is too small");
 
+				bool? AdoptPartiallyPlayableIntoLargest(CPos cpos, bool first)
+				{
+					if (first)
+						return false;
+					else if (regionMask[cpos] == largest.Id || playability[cpos] != PlayableSpace.Playability.Partial)
+						return null;
+
+					regionMask[cpos] = largest.Id;
+					largest.Area++;
+					return false;
+				}
+
+				// Adopt any partially playable tiles connected to the largest region into the largest region.
+				// This avoids potentially debris-filling connected oceans for mods where water is unplayable.
+				CellLayerUtils.FloodFill(
+					regionMask,
+					map.AllCells.Where(cpos => regionMask[cpos] == largest.Id).Select(cpos => (cpos, true)),
+					AdoptPartiallyPlayableIntoLargest,
+					Direction.Spread4CVec);
+
 				if (param.DenyWalledAreas)
 				{
 					// Beach tiles are particularly problematic. If they're for unplayable bodies