bgra format. issue 254
opengl display rgb with alpha correctly. qml can't
add a convenience class to extract frame at a given time
async frame extract support

TODO: why do event and slots not works?
qml: add Preview item
avoid preview seeking too frequent etc.
Merge branch 'preview'
preview: fix error decoding key frame after seek
avoid frequent seeking
wrong seek position VideoFrameExtractor
support both ffmpeg/libav for debian package
add -dbg to debian packages
clean qmake detection on debian package clean rule
support for sequential QIODevices