Closes #540

This is the same song, different verse of the recently merged #1735. The XITS Math parentheses have a low baseline.

However, the baseline plays no role in math layout:

TeXbook, Appendix G, Rule 19 (deep in the paragraph): "Shift the delimiters up or down so that they are vertically centered with respect to the axis."

XeLaTeX and MS Word have no trouble with delimiters with XITS Math.

This PR ensures that delimiters (which arise in vector, matrix, binomial coefficient, and LR constructions) are centered on the axis.

There were 9 test files that had changes. All were very minor (2% or less pixel variations) associated with minor shifts associated with delimiters.

#set text(size:12pt)
#show math.equation: set text(font:"XITS Math", fallback:false)
$
( A / (sum_c^d k) )
$

Before

After