Fix corrupt initial frames https://github.com/wang-bin/QtAV/issues/70

There is a race condition where AVDemuxThread starts queueing packets
before AVThread::resetState is called from VideoThread. When AVThread::
resetState gets called, all collected frames get dropped. This patch
waits on a conditional variable for AVThreads to start properly.
Merge pull request #100 from e3c/master

Fix corrupt initial frames https://github.com/wang-bin/QtAV/issues/70