glyph order is surprisingly slow and blocks everything

#443 shows that the glyph order task blocks the entire system because:

1. It can't run until FE work is complete
1. It blocks all BE work

Reproduction:

```shell
$ rm -rf build/ perf.data && cargo flamegraph -p fontc -- --source ../OswaldFont/sources/Oswald.glyphs --emit-ir false --emit-timing

# ../FlameGraph assumed to be a clone of https://github.com/brendangregg/FlameGraph
$ perf script | ../FlameGraph/stackcollapse-perf.pl | grep GlyphOrderWork | ../FlameGraph/flamegraph.pl > glyphorder-flames.svg
```

`build/threads.svg` shows the glyph order blockade:


The focused flamegraph gives some sense of where the time goes:


It's not the biggest blocker, just notable for stalling all the things.