Add overline option for Table rows

[akx]

• Closes no issue, I just needed this for an app and figured it could be useful.
• I have followed the instructions in the PR template

This PR adds an overline option for egui_extras::TableRow, which is useful for visually grouping rows. The overline consumes no layout space.

A screenshot of the demo app, showing every 7th row getting an overline.

[akx]

Rebased.

[emilk]

Sorry for the slow review - needs a merge fix for the screenshot test

[akx]

@emilk No worries – thank you for Egui, it's awesome! Rebased + updated snapshot + added your suggestion.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5637-table-overline
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

I would expect the line to be centered between rows, but instead it seems to be shifted to the bottom? Is this intentional?

[akx]

I would expect the line to be centered between rows, but instead it seems to be shifted to the bottom? Is this intentional?

It's currently drawn on the zeroth pixel row of the table row it's overlining. I wasn't convinced that drawing it at a half-pixel offset upwards would be a good idea...