Google Code-in

We just updated libgpac, which is the library that does all the hard work when it comes to parsing .mp4. However on top of updating libgpac we also need to refresh our integration with it. In the specific issue mentioned below, we are not finding the subtitles, even though gpac is telling us that there's two subtitle tracks. We're just ignoring them because (as in CCExtractor's code, in mp4.c) don't know about this track type.