message: [libterminal] Screen: adds const version for at(Coordinate)
diff: @@ -438,6 +438,13 @@ class Screen {
  return absoluteAt(Coordinate{y, _coord.column});
  }
  + // Gets a reference to the cell relative to screen origin (top left, 1:1).
  + Cell const& at(Coordinate const& _coord) const noexcept
  + {
  + auto const y = static_cast<cursor_pos_t>(currentBuffer().savedLines.size() + _coord.row - scrollOffset_);
  + return absoluteAt(Coordinate{y, _coord.column});
  + }
  +
  // Gets a reference to the cell relative to screen origin (top left, 1:1).
  Cell const& at(cursor_pos_t _rowNr, cursor_pos_t _colNr) const noexcept
  {
  