[tex-live] Support extensible accents

[Khaled Hosny]

Hi all,

Currently XeTeX does not support extensible accents (just like TeX)
which leads unicode-math to doing some ugly hacks to support \overbrace
etc. The attached patch provides support for extensible accents (by
adapting some commented out code and taking some inspirations from
luatex implementation).

The output is nearly identical to luatex's except that luatex takes
account of \delimiterfactor and \delimitershortfall when calculating the
desired width of the accent, but luatex behaviour does not confirm to
Office implementation nor \overbrace behaviour in plain tex, so I didn't
follow it.

The second patch does some cosmetic changes to better reflect the new
job of build_opentype_assembly(). Both patches are to be applied after
the previous ones.

Regards,
 Khaled
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