bug#71454: 30.0.50; Performance issues with font selection

[Kai Ma <justksqsf@gmail.com>]

Recently, I have the need of editing files with extraordinary ranges of
Unicode code points, and the performance problem with font selection
becomes too obvious to ignore.

I have currently (length (font-family-list)) = 582 font families
installed. And whenever I input some ununsual characters, Emacs will
freeze for seconds until I am able to do anything else.  Worse, the
freeze delay for each character will add up.  And whenver the face
changes (including hl-line-mode), or I switched to another buffer for
some time, there will be a delay again.

I'm pretty sure this is due to font selection, because Emacs won't
freeze if I configure manually the fallback fonts for each 'exotic'
script I encounter.

For testing, there are some such characters in my file:

〡〢〣〤〥〦〨〩〸〹〺

[ū, ú, ű, ǔ, ù, ȕ, û, ŭ, ȗ, ü, ǖ, ǘ, ǚ, ǜ, ů, ũ, ᵤ, ᵘ, ʉ, ᶶ, ủ, ų, ṷ, ụ,
ṳ, ṵ, ư, ʊ, ᶷ, ᵿ, ᶙ, ṻ, ṹ, ứ, ừ, ữ, ử, ự, ꭒ, ꭟ, ꝸ, ꭎ, ꭏ, ᴝ, ᵙ, ᴞ]

[ü, ǖ, ǘ, ǚ, ǜ, ṽ, ᵛ, ᵥ, ṿ, ꝟ, ʋ, ᶹ, ᶌ, ⱴ, ⱱ, ỽ, ʌ, ᶺ]

[Ü, Ǖ, Ǘ, Ǚ, Ǜ, Ṽ, ᴠ, ⱽ, Ṿ, Ꝟ, Ʋ, Ỽ, Ʌ ]

[ ₀, ₁, ₂, ₃, ₄, ₅, ₆, ₇, ₈, ₉, ₊, ₋, ₌, ₍, ₎, ‸, ᴀ, ₐ, ᴁ, ʙ, ᴃ, ᵦ, ᴄ, ᴐ, ᴒ, ᴅ, ᴆ, ᴇ, ₑ, ₔ, ᵩ, ɢ, ʛ, ᴦ, ᵧ, ʜ, ₕ, ɪ, ᵻ, ᵢ, ᴊ, ⱼ, ᴋ, ₖ, ʟ, ₗ, ᴌ, ᴧ, ᴍ, ₘ, ꟺ, ɴ, ᴎ, ₙ, ᴏ, ₒ, ɶ, ʘ, ᴓ, ᴑ, ᴘ, ₚ, ᴨ, ᴪ, ʀ, ᵣ, ᴙ, ʁ, ᴚ, ᵨ, ₛ, ᴛ, ₜ, ᴜ, ᵤ, ᵾ, ᴠ, ᵥ, ᴡ, ₓ, ᵪ, ʏ, ᴢ, ᴣ ]

[ 五, 伍, ₅, ⁵, Ⅴ, ⅴ, ⑤, ➄, ❺, ➎, ⓹, ⑸, ⒌, ５, ㊄, ㈤, 㐅, 㠪, 𠄡 ]