TerrainFillMesh unit tests fail under Firefox Headless (and maybe others)

Under Firefox Headless (and possibly other options) a bunch of `TerrainFillMeshSpec` unit tests fail with the below error:

```
renderState.lineWidth is out of range. Check minimumAliasedLineWidth and maximumAliasedLineWidth.
```

Not sure if this is exposing a bug in our code we previously missed, or if the test is just bad.

Start by running the TerrainFillMeshSpec unit tests under Firefox Headless and capture which cases produce the renderState.lineWidth error. Read the TerrainFillMesh test setup and the relevant rendering code to determine whether the failure is caused by the implementation or by invalid test expectations. Done means the affected tests pass reliably under Firefox Headless and other tested options.

Written by the indexing model from the issue text.