This PR fixes #6512.

It doesn't solve David's issue at large, which we've expressed reluctance about, but it does fix the example pointed out.
From #6512 (comment):

Now for geom_density in particular this is a bit surprising because it is a polygon under the hood so it present itself as a line but is in fact a polygon. Maybe we can do something there.

This changes the border parametrisation to line parametrisation.