[css3-page] default value of 'fit-position'

The default value of 'fit-position' is currently 'top left' (or its
percentage equivalent).  I think that it should instead be 'center',
for the following reasons:

1. For the 'slice' and 'hidden' values, I think photos are generally
more likely to appear reasonable when the center of the photo is
shown than when the top left corner is shown.

2. For all values, it has less of a bias towards working well with
left-to-right languages and badly for right-to-left languages.

-David

-- 
L. David Baron                                <URL: http://dbaron.org/ >
           Technical Lead, Layout & CSS, Mozilla Corporation