The logic here is needed by InlineNode too. Moving it into a
`paint_all_borders()` function makes it available to them both, as well
as anyone else who wants it. :^)
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
