Refactor WidgetIndentsHighlightingPass and/or add tests here

This logic in `paint` is finicky and very hard to follow; I suspect we could do better.

As a first step, perhaps extract the contents of `paint` into a separate function `generateLines` that returns a collection of lines to paint (with x, y positions), then have `paint` only render these.

e.g.:
```
class LinesToPaint {
int startX;
int startY;
...
}
```

in `paint`:
```
LinePainter2D.paint(
g2d,
lineToPaint.startX,
lineToPaint.startY,
...
);
```

And then we can at least test `generateLines` for regressions.