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/tgp.cpp

@@ -884,8 +884,6 @@ static void HeightMapNormalize()
 	if (_settings_game.game_creation.variety > 0) {
 		HeightMapCurves(_settings_game.game_creation.variety);
 	}
-
-	HeightMapSmoothSlopes(I2H(1));
 }
 
 /**
@@ -1006,8 +1004,6 @@ void GenerateTerrainPerlin()
 		}
 	}
 
-	IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);
-
 	FreeHeightMap();
 	GenerateWorldSetAbortCallback(nullptr);
 }