AVDemuxer returns Packet not ptr. remove useless members

return object is more reasonable.
removed members were added long time ago and not ever used and not
reasonable.
better clock correction
always set clock initial value. stop correction timer if not playing

otherwise if use external clock the first value() maybe <0 and will seek
to to startPosition(). we can check mediaStartPosition() to avoid seek.
but set initial clock value is better
notify timer interval is not const. depends on duraiton and fps

small duration video can update progress smoothly
remove useless members in statistics. better fps estimate
Merge remote-tracking branch 'upstream/master'