vo: remove unused 'preview' member.
store data ref in writeData(). convertData() is not pure
gdi: inherits VideoOutput. d.data must not be modified when painting

TODO:
lock and store data in AVOutput::convertData()? code will be more clean
gdi: StretchBlt() to improve performance. fix mem leak

performance improved a lot, half cpu usage on my pc.
TODO: store mem bitmap and hdc in private
gdi: refine code

TODO:
can offscreen bitmap be created only once? how to copy image data?
Merge branch 'directx'
move write() to protected
add an example for each renderer so we can compare the performance
videowall: help
renderer: aspect ratio support. default is keep the original

TODO:
more understandable api names
names change, warning fix, move some public members to private

I think QtAV is not used widely today, so api change does not matter
move useQPainter() to base class VideoRenderer
correct var name
let all renderers support aspect ratio
direct2d: crash fixed. always lock is easier, and will not forget to
unlock