Add fallback information to font-data of newer webfonts

From https://github.com/mathjax/MathJax-node/issues/78: the fontdata for all the web fonts (other than the default TeX font) is missing fallback information.

The data about where to look for characters in a given font does not include the fallback to the main fonts if the character isn't in the given font.

E.g., https://github.com/mathjax/MathJax/blob/master/unpacked/jax/output/SVG/fonts/STIX-Web/fontdata.js#L181 should read

fonts: [SANSSERIF,MAIN,MONOSPACE,LATIN,ALPHABETS,MARKS,
        ARROWS,OPERATORS,SYMBOLS,SHAPES,MISC,VARIANTS,SIZE1],

And similarly for other variants.