Function is_glyph_covered 

pub fn is_glyph_covered(chain: &FontChain<'_>, ch: char) -> bool

Whether ch renders as its own glyph in chain: covered by some font in the chain, not the substituted notdef fallback.

A character no font in chain can draw at all (not even the fallback – see FontChain::resolve) is also “not covered”: it draws nothing, which is no more ch’s own glyph than the solid block is.