VAAPI: separate module from avcodec

The VLC avcodec plugin links with vaapi-x11. This makes the entire avcodec plugin dependent on X11. There has been a lot of confusion on Debian/Ubuntu with the move of avcodec from vlc-nox to vlc in headless VLC use cases.

libavcodec proper only needs libva, not libva-x11. The VLC plugin should exhibit the same dependency. There are two ways to achieve this:

a plugin capability for lavc/vlc HW back-end (VAAPI, DxVA2, ...),
a plugin capability for VAAPI windowing system (X11, EGL...)

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