Fix missing CJK/Indic text on Linux with lazy font fallback

[nibbbl]

Resolve fallback fonts per-glyph from FontConfig on demand instead of a truncated static list. This is a general improvement over the prior system, which simply slurped up 40 sorted fonts from the OS, causing missing characters on systems with many fonts. The newer system should result in less fonts being loaded, which is a nice bonus.

Closes #5991

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