[css-masking] 'mask-mode' must not have a value called 'auto'

https://drafts.fxtf.org/css-masking-1/#the-mask-mode should not have
a value called 'auto' since it makes parsing the mask shorthand
ambiguous, and https://drafts.fxtf.org/css-masking-1/#the-mask-size
(which takes the same values as background-size) already has a value
called 'auto'.  The current 'auto' value needs a new name that isn't
used by any of the other properties in the shorthand.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)