Text line FontRunIter fails on JetBrains Mono and combining characters

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.

Start by reproducing the FontRunIter case described in the issue with U+0061 followed by U+0306 using JetBrains Mono. Trace how the combined cluster selects a fallback when the font lacks the combining mark, and verify that the cluster is shaped with JetBrains Mono when it supports the composed result.

Written by the indexing model from the issue text.