vaapi: render osd and eosd into single subpicture

* VA can associate only one subpicture with each surface.
* Now vaapi renders subtitles at the screen resolution.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
vaapi: start swithing to the new vo api

let's blow it up

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>