def render_to_file(filename, format = nil)
      if format
        __render_to_file_with_format__(filename, format)
      else
        __render_to_file__(filename)
      end
      return File.exists?(filename)
    end
def render_to_file(filename, format = nil)
      if format
        __render_to_file_with_format__(filename, format)
      else
        __render_to_file__(filename)
      end
      return File.exists?(filename)
    end
Renders the map to a file. Returns true or false depending if the
 render was successful. The image type is inferred from the filename.
 @param [String] filename Should end in one of "png", "jpg", or "tiff" if format not specified
 @param [String] format Should be one of formats supported by Mapnik or nil (to be guessed from filename)
 @return [Boolean]