qml: emit playbackStateChanged after metaData is set
avoid out of sync after seek

before pakcet queue.clear(), a/v thread may takes 1 packet and
later compares to new packets after seek. the diff will be large. just
ignore once
when seeking, can not contunue without decoding video.
skip decoding if video too slow
frame drop and skip render if video too slow. hi fps video is better
ensure video not later than 2s
add QtAV to ffmpeg extra version
position can not be before startPosition
player: img seqences 01
Merge remote-tracking branch 'upstream/master' into imgseq
player: reuse repeat zone

after stop now can be used repeat zone again