[fix] Allow the use of external subtitles for DVDs

as title says.

there are several forums threads that mention this issue:
http://forum.xbmc.org/showthread.php?tid=107211
http://forum.xbmc.org/showthread.php?tid=153538

The root cause that this didn't work was twofold:
a) pts incorrect because the offset wasn't added
b) the subtitle was ignored in case of a DVD

fix is quite easy and does not affect other container formats. The only thing I had doubts about was the change to the pts value, but also that is fine with all materials I tested.