Feature: Place rocks on "too steep" tiles when fixing slopes. (#13462)

When generating maps or loading heightmaps, the terrain height is altered to prevent slopes that can't be represented.

During this, there is now a chance of these tiles being turned into a rocky tile.

Chance of placing rocks is based on the height. This gives a rocky mountain appearance without affecting all peaks.

src/clear_cmd.cpp

@@ -165,7 +165,7 @@ static void TileLoopClearAlps(TileIndex tile)
 		/* Below the snow line, do nothing if no snow. */
 		/* At or above the snow line, make snow tile if needed. */
 		if (k >= 0) {
-			MakeSnow(tile);
+			MakeSnow(tile, IsClearGround(tile, CLEAR_ROCKS) ? GetClearDensity(tile) : 0);
 			MarkTileDirtyByTile(tile);
 		}
 		return;