Sets the font pointsize to use when annotating with text.
Inheritance: IDrawable
    private MagickImage CreateImage(int? density)
    {
      MagickImage image = new MagickImage(MagickColors.Purple, 500, 500);
      DrawableFontPointSize pointSize = new DrawableFontPointSize(20);
      DrawableText text = new DrawableText(250, 250, "Magick.NET");

      if (!density.HasValue)
        image.Draw(pointSize, text);
      else
        image.Draw(pointSize, new DrawableDensity(density.Value), text);

      image.Trim();

      return image;
    }