computeStringSize does not recognise calligraphic, script variants

[xworld21]

...and presumably any other variant whose XMath representation is ascii+variant rather than straight Unicode.

For example:

\documentclass{article}
\usepackage{amsfonts}
\usepackage{mathrsfs}
\begin{document}
  \newbox\test

  \setbox\test\hbox{$A$} sizes: \the\wd\test\ x \the\ht\test\ x \the\dp\test

  \setbox\test\hbox{$\mathcal{A}$} sizes: \the\wd\test\ x \the\ht\test\ x \the\dp\test

  \setbox\test\hbox{$\mathscr{A}$} sizes: \the\wd\test\ x \the\ht\test\ x \the\dp\test
\end{document}

LaTeXML returns identical sizes for the three characters, even though they are quite different.