libharu：HPDFAnnotation#set_highlight_mode

everyday Ruby 
#!/usr/bin/env ruby

require "hpdf"

pdf  = HPDFDoc.new
page = pdf.add_page
page2 = pdf.add_page
dst = page2.create_destination()

y = 0
[HPDFDoc::HPDF_ANNOT_NO_HIGHTLIGHT,
 HPDFDoc::HPDF_ANNOT_INVERT_BOX,
 HPDFDoc::HPDF_ANNOT_INVERT_BORDER,
 HPDFDoc::HPDF_ANNOT_DOWN_APPEARANCE
].each do |mode|
  # NG
  #a = page.create_text_annot(0, y, 100, y + 100, "foo", pdf.get_current_encoder)

  a = page.create_link_annot(0, y, 100, y + 100, dst)
  #a = page.create_uri_link_annot(0, y, 100, y + 100, "http://libharu.org/")
  a.set_highlight_mode(mode)
  y += 100
end

pdf.save_to_file(File.basename($0, ".rb") + ".pdf")