Make dashed border-style work correctly

Merged Arnaud B. requested to merge (removed):wip/arnaudb/fix-dashed-border into gtk-3-24

There’s a short-path done for focus rectangles, but it can be taken in other conditions, and then fail occasionally to render a dashed line if the border-width is too big.

Screenshot of the problem

It should probably be fixed also in master, but 1) I cannot test and 2) I don’t know if there’s an easy way to do that with Gitlab.

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