Update libass to 0.11.2

Now master has the necessary changes to accommodate the new libass I
thought I would update it.
Fixes xbmc crashing with some 10bit MKV's that have dodgy subtitles, as
reported here..
http://boxeed.in/forums/viewtopic.php?f=13&t=858
Now fixed, but file takes a while to start because of the subtitle
errors.