make ffmpeg demuxer observe pmt changes

@elupus
I have borrowed the idea of registering a callback for pmt changes from mythtv. Streams which are not in the current pmt are set inactive in order to be ignored by OpenDefaultStreams.
I tested with vdr recordings which have a pat/pmt before every GOP. pvr clients which use this demuxer need to send  pat/pmt at least when there are changes.