With the necessary pixel formats defined, we can now expose support for
the remaining 10/12bit combinations that VAAPI can handle.

Specifically, we are adding support for:

* HEVC
** 12bit 420
** 10bit 422
** 12bit 422
** 10bit 444
** 12bit 444

* VP9
** 10bit 444
** 12bit 444

These obviously require actual hardware support to be usable, but where
that exists, it is now enabled.

Note that unlike YUVA/YUVX, the Intel driver does not formally expose
support for the alphaless formats XV30 and XV360, and so we are
implicitly discarding the alpha from the decoder and passing undefined
values for the alpha to the encoder. If a future encoder iteration was
to actually do something with the alpha bits, we would need to use a
formal alpha capable format or the encoder would need to explicitly
accept the alphaless format.