[Fontconfig] How to block glyph fallback?

[Wendell P]

I need to block glyph fallback in specific applications on Linux, so
that if the chosen font lacks a glyph, just the "missing glyph" glyph is
shown.

The apps are text editors based on GtkTextView, so I searched the GTK
docs but couldn't find any way to control this feature. Can fallback
only be controlled through fontconfig?

I would like to be able to disable fallback both per application and per
user.

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