Linux: CJK/Thai/Indic text does not render even with matching system font installed

[nibbbl]

Environment

Any recent viewer build on Linux (Official, Firestorm, etc.)

Description

On linux, text in Japanese, Chinese and other non-Latin scripts renders as empty squares / tofu boxes even when the system has the correct font installed (ex. Noto Sans CJK.)

The cause of this appears to be that the viewer uses a max_font_count_cutoff of 40 when building the list used by LLWindowSDL::getDynamicFallbackFontList(), which cuts off the list before a valid font is found. (Fonts like CJK can often appear at indexes of 180th or so.)

Possible Solutions

There's a few possible approaches to address this issue:

1. Expand the limit - Bump the limit from 40 to a high value such as 400
2. Explicitly query FontConfig for its best match for a fixed set of major scripts
3. Lazy-load on demand - pull in fonts on demand if characters are missing

Reproduction steps

Given: You are running a Linux build of Second Life

1. Log in
2. View Japanese/Chinese/Thai font

Expected: The font should be rendered
Actual: The font is rendered with placeholder boxes

Screenshots

Broken text in nearby chat

Broken text in the Friend List

[nibbbl]

(Oops, I accidentally closed this issue rather than #5992 --would appreciate if a maintainer would reopen it. Thanks!)