Make sure stretchy embellished operators use the proper scaling when determineing their widths

[dpvc]

This PR fixes a problem reported by Peter where some horizontally stretched characters are stretched to the wrong length. For example:

\bbox[red]{\begin{smallmatrix}
 - \otimes _k tk \\
  \xleftarrow {\hspace{50pt}}\\
  \xleftarrow {\hspace{50pt}}\\
  \xleftarrow {\hspace{50pt}}\\
  \operatorname {Hom}_k(tk,-)
\end{smallmatrix}}

in SVG output. An incorrect relative scaling value was being used (with an embellished operator, the relative scaling of the core has to include all the nesting parent elements).