Evince Highlighting support

So this is a quick update on Giselle post on SoC. (see here) There you could see a screencast of highlighting working but with a big lag. It turns out that the lag was related to a memory leak on the cache engine that only was triggered when doing the "animation" in the drag-and-drop of the highlighting annotation. Giselle luckily fixed this already, and I managed to spend some time adding new API to poppler to render the annotation directly, without need of reloading the page as we were force to before. Thus, we get an even smoother animation, as you can see here.