[css3-writing-modes] rename 'text-orientation' initial value to 'mixed-right'

After reviewing Microsoft's proposal for UTR50, Koji and I thought the name
'mixed-right' was better than 'upright-right' or the previous 'vertical-right'.
So I'm making this change, unless people object. :)

<del>text-orientation: vertical-right</del>
<del>text-orientation: upright-right</del>
<ins>text-orientation: mixed-right</ins>

~fantasai