message: Use hardcoded 0 for surface level when serializing mine
diff: @@ -401,7 +401,7 @@ void TileMap::serialize(NAS2D::Xml::XmlElement* element)
  for (const auto& location : mMineLocations)
  {
  - auto& mine = *getTile({location, TileMapLevel::LEVEL_SURFACE}).mine();
  + auto& mine = *getTile({location, 0}).mine();
  mines->linkEndChild(mine.serialize(location));
  }
  