World: fixed every chunk having terrain saved at least once, even if unmodified

setPopulated() sets dirty flags on the chunk, causing the autosave sweep
to think they've been changed when they haven't. We now pass
terrainPopulated to the constructor to avoid this ambiguity recurring in
the future.

src/world/format/io/region/LegacyAnvilChunkTrait.php

@@ -88,13 +88,13 @@ trait LegacyAnvilChunkTrait{
 			$biomeArray = $makeBiomeArray($biomesTag->getValue());
 		}
 
-		$result = new Chunk(
-			$subChunks,
-			$biomeArray
-		);
-		$result->setPopulated($chunk->getByte("TerrainPopulated", 0) !== 0);
 		return new ChunkData(
-			$result,
+			new Chunk(
+				$subChunks,
+				$biomeArray,
+				null,
+				$chunk->getByte("TerrainPopulated", 0) !== 0
+			),
 			($entitiesTag = $chunk->getTag("Entities")) instanceof ListTag ? self::getCompoundList("Entities", $entitiesTag) : [],
 			($tilesTag = $chunk->getTag("TileEntities")) instanceof ListTag ? self::getCompoundList("TileEntities", $tilesTag) : [],
 		);