corner-shape: account for diagonal overlaps

The previous mechanism for rendering corner-shape with border
was to intersect two diagonal paths.
This assumes that diagonal corners cannot intercept.

However, there are extreme conditions (added in WPT) where
diagonal concave corners CAN overlap.

So instead of intersecting two paths, this intersects 4 paths,
one for each corner.

We can optimize this in the future by trimming beziers instead
of intersecting the whole path, but going with correctness first.

Bug: 399449172
Change-Id: Id8e14524633074bb8b69a11c1f99e4076de7d015
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6445851
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1445754}