add some signals in AVClock

updateExternalClock can be non-inline. it is called outside and not frequently
TODO: updateExternalClock(const AVClock& clock)
fix crash and video blocking when seeking

avcodec_flush_buffers() immediatly after seeking may cause a crash or
blocking the thread, but
i don't know why.
TODO: flush in AVThread when got a flush packet
add clock control test
ipp works. color is wrong
Merge branch 'ipp' of github.com:wang-bin/QtAV into ipp

Conflicts:
	src/ImageConverterIPP.cpp