[XeTeX] Fixing displacement of big operator's limits with some fonts

[Khaled Hosny]

Hi all,

A while ago, it has been reported that with certain version of Neo Euler
font, summation limits were displaced[1] and the issue were later fixed
by changing DisplayOperatorMinHeight math constant.

Now I found the root of this. If DisplayOperatorMinHeight is higher than
the height+depth of the display variant of a big operator, the limits
are displaced (which was the case with summation in Neo Euler), this is
because the code path in this case does not update the metrics of the
constructed box to that of the last found glyph and keep using the
metrics of the base glyph.

The attached patch fixes this issue for me, as demonstrated by the
attached files.

[1] http://tug.org/pipermail/xetex/2010-October/019007.html

Regards,
 Khaled

P.S. The limits of the first formula in the page being set in script
style, but it seems to be an unrelated issue, and happens with other
fonts as well.
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