bug 837461 - backout parts 2 and 3 of bug 808288 because pdf.js may paint missing chars to canvas if it failed to load fonts, which looks really ugly; better to skip them for now. r=bas a=akeybl

--HG--
extra : transplant_source : %08%D96r%24%C5%0A%FB%B1%E6%E0%03q%D1s%3C%24%2B%26%1F