vp9: write uveob as 16-bit value for 16x16/32x32 transforms.

This fixes make fate-vp9-00-quantizer-01 THREADS=2.
Merge commit '6d69f9f37689c999815a65a2d99999fad3a41705'

* commit '6d69f9f37689c999815a65a2d99999fad3a41705':
  vp9: write uveob as 16-bit value for 16x16/32x32 transforms.

Merged-by: Michael Niedermayer <michaelni@gmx.at>