xv30be is an obnoxious format that I shouldn't have included in the
first place. xv30 packs 3 10bit channels into 32bits and while our
byte-oriented logic can handle Little Endian correctly, it cannot
handle Big Endian. To avoid that, I marked xv30be as a bitstream
format, but while that didn't produce FATE errors, it turns out that
the existing read/write code silently produces incorrect results, which
can be revealed via ubsan.

In all likelyhood, the correct fix here is to remove the format. As
this format is only used by Intel vaapi, it's only going to show up
in LE form, so we could just drop the BE version. But I don't want to
deal with creating a hole in the pixfmt list and all the weirdness that
comes from that. Instead, I decided to write the correct read/write
code for it.

And that code isn't too bad, as long as it's specialised for this
format, as the channels are all bit-aligned inside a 32bit word.