Call paint_border() recursively, once for the outer line, and once for
the inner one. This is done in a lambda so that we can reuse it for a
couple of other line styles.
Border-radius behaviour doesn't match other browsers, and goes a bit
haywire in some cases. I've left some FIXMEs for someone who
understands the maths here better than I do. 😅
The LineStyle handling is moved to the start of the function, to avoid
unnecessary work.
