On Ubuntu 25.10, which includes Noto Color Emoji, I was seeing that the
FontsDemo would assert when attempting to render non-emoji text using
this font. It appears that FontConfig will tend to return Noto Color
Emoji when this family name is passed, even though the font may not
cover the required character set.
The new strategy is to use FontConfig as before, but then to check the
resolved font for coverage of the string. If the resolved font still
can't render the string, we relax the font matching constraints by
removing the family name from the pattern, then try matching again.