initial value = 0 in AVClock.reset()

test: play v4l2 camera, then a normal file
error code change. can set custom description

add some enums from qtmm
decoder add error signal
qml error handling

TODO: emit NoError in AVPlayer when it start?
fix build error using libav
check network error
AVClock is much more accurate without audio

reduce accumulative error of
QElapsedTimer.restart()
al: wrong format for 8bit 7, 8 channels
add qml Video.error & update plugins.qmltypes
add log level support(release=Logoff). qt log hack

TODO:
now only qDebug. add qWarning etc.
ffmpeg logging should support QT_MESSAGE_PATTERN
API in QML
more complete qt logging support

TODO: Qt5 QLoggingCategory
fix wrong logging logic etc

log level takes affect for ffmpeg and libass log
TODO:
remove qDebug() in headers
different function type error(android)
misuse export macro
new: orientation support in qml

only for opengl mode
TODO:
point mapping, roi(use a matrix in
VideoRendererPrivate?)
test other classes
qml: keep paybackState values the same as qtmm
fix log crash. issue 310

QDebug* was deleted at ~QtAVDebug() after
QtAVDebug::debug() returns a
copy of QtAVDebug
add decoder dynamic properties used by UI
orientation range is [0, 360)
support both ffmpeg/libav for debian package