Inline fallback fonts should be sized to main font’s EM height, not line height

[astiob]

When a glyph is missing in the middle of a run and a fallback font is selected for it, then instead of scaling usWinAscent + usWinDescent to \fs for the fallback font in the same manner as we do for the main font, we should scale the fallback’s units_per_EM to the same amount of pixels that the main font’s units_per_EM occupy after applying the main font’s scale, i. e.:

fallback_font->scale =
    fs / (main_font->usWinAscent + main_font->usWinDescent)
    * main_font->units_per_EM / fallback_font->units_per_EM;

This seems to be how GDI does it (see test in #706 (comment), although more comprehensive tests are welcome), and this would also help with fallback between Noto Sans and Noto Sans Arabic in mpv-player/mpv#13396.