def to_png
      data = ""
      to_png_stream Proc.new { |chunk| data += chunk }
      yield data if block_given?
      data
    end
def to_png
      data = ""
      to_png_stream Proc.new { |chunk| data += chunk }
      yield data if block_given?
      data
    end
Render the data to a blob of png data.