Add upipe_av_pixfmt

Mapping between avutil pixelformat and
upipe ubuf_pic planes.
upipe_avcdv: use upipe_av_pixfmt
upipe_sws: complete refactoring

Cleaner, use upipe_av_pixfmt
examples/glxplay: remove yuvrgb

Drop yuvrgb code, use upipe_sws (which now supports multiformat)
upipe_sws: improve debug
Update gitignore
upipe_avcdv: fix audio decoding, add audio attr
Add upipe-osx, upipe_osx_audioqueue_sink (WIP)

upipe-osx will contain macosx specific pipes.
upipe_osx_audioqueue_sink is the AudioQueue upipe
sink module used for playback.
Build upipe-osx if appropriate