[STIX-web] bold-italic glyphs no longer found (in 2.7.5)

E.g., <math><mi mathvariant="bold-italic">x</mi></math> fails to locate a glyph in the STIX-web fonts (in both SVG and HTML-CSS).

Possibly a regression after #1923 (closed) (since 2.7.4 uses STIXWEBMAINBI).