Unicode characters beyond the BMP confuse Markdeep's diagrams

diagram-vs-Unicode.md.html

The bold capital Sigma (U+1D6BA) in the diagram below will not
be rendered.  Since it occupies two indices in the extracted
diagram string, grid width computation is off by one (see the
spurious `*` rendered in the resulting document):

*******
* x𝚺x *
* xxx *
*******

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