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?