Trying to render a subscript ends up with bold font

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Using a file like
```

```

and `./resvg --sans-serif-family "Liberation Sans" simple_subscript_bold.svg simple_subscript_bold.png` gives a warning "Warning (in usvg_text_layout:1398): Fallback from Liberation Sans to Noto Sans Mono." and result like

However specifying `./resvg --sans-serif-family "Noto Sans Mono" simple_subscript_bold.svg simple_subscript_bold.png` doesn't give any warnings and also doesn't render the text in bold:

I suppose the problem is that Liberation Sans doesn't have the codepoint 2083 (subscript 2) available: `fc-query --format='%{charset}\n' /usr/share/fonts/truetype/liberation/LiberationSans-Regular.ttf` ⇒ `… 207f 20a3-20a4 …` and Noto does (`2074-208e`). But I would expect then that a regular weight fallback would be used, and preferably a warning why the fallback was used when the requested font does exist.

Reproduce the issue with the supplied SVG and the two resvg commands, then start at the usvg_text_layout warning path mentioned in the report. Trace how missing glyphs select a fallback font and how its weight is chosen. Done means Liberation Sans fallback renders the subscript without unintended bold styling and the fallback behavior is explained by an appropriate warning.

Written by the indexing model from the issue text.