rework video handling
in particular:
- Video output handling has been changed. Codecs are now assigned a format that
the decoders have to decode to (I420 for H263/VP6, RGB for screen video).
The decoded video is then manually colorspace converted by Swfdec.
(NB: This is currently only implemented using FFmpeg.)
- Decoded video images are returned in a much more verbose way.
- Using these new features, VP6 alpha was implemented.
Another thing this patch should ease is transition to colorspace conversion by
Cairo (or at least by GL).