Improve PGS parsing: the "state" field is not relevant to us,

Improve PGS parsing: the "state" field is not relevant to us,
the object number is, it determines whether we should continue
parsing the presentation description and whether we should
clear the subtitles on the next display command.
Based on patch by Mark Goodman [mark goodman gmail com]

Originally committed as revision 25682 to svn://svn.ffmpeg.org/ffmpeg/trunk