Work around combining characters being handled as 0-width automatically by some browsers

[dpvc]

This PR fixes a problem where some accents are misplaced. Combining accents like U+0302 are handled in some fonts by making zero-width characters that have negative left bearing (so they extend to the left over the character that precedes them). Modern browser will position combining characters automatically, whether they are zero-width or not (and they do vertical positioning so that accents are placed over both lower-case and capital letters properly).

MathJax expects the browser to respect the widths of the characters in the font, so when the browser makes it zero-width automatically, MathJax misplaces the accent. This PR makes sure that the accents are properly placed regardless of whether the character is zero-width or not, and regardless of whether the browser forces zero-width for ones that are non-zero in the font itself. This is done by setting the width of the <mjx-c> to the expected width of the character (and removing the padding that used to make it that wide), and using text-align: right so that a zero-width character will move to the right and so be placed properly, while a full-width character will be placed in the right place to begin with.

[zorkow]

lgtm.