Text line FontRunIter fails on JetBrains Mono and combining characters

[tonsky]

Copied from https://github.com/JetBrains/skija/issues/106

Given this:

a    U+0061  LATIN SMALL LETTER A
 ̆     U+0306  COMBINING BREVE

JB Mono supports a and ă but not ̆. That leads to whole cluster to fall back, but it should be shaped using JB Mono instead.