Codechange: Use std::tuple for slope functions with two return values

src/void_cmd.cpp

@@ -29,8 +29,7 @@ static int GetSlopePixelZ_Void(TileIndex, uint x, uint y, bool)
 {
 	/* This function may be called on tiles outside the map, don't assume
 	 * that 'tile' is a valid tile index. See GetSlopePixelZOutsideMap. */
-	int z;
-	Slope tileh = GetTilePixelSlopeOutsideMap(x >> 4, y >> 4, &z);
+	auto [tileh, z] = GetTilePixelSlopeOutsideMap(x >> 4, y >> 4);
 
 	return z + GetPartialPixelZ(x & 0xF, y & 0xF, tileh);
 }