corner-shape: compute path given curvature & offset

To render borders & shadows at constant thickness along the superellipse
(or gradual if interpolating between two border widths), we do the
following:

- Keep the original border-rect as an "origin rect", so that we can
  reference it,
  when outsetting/insetting.
- When rendering the "target rect" (the rect with the inset/outset),
  we extend
  the radius of the origin rect by the offset, keeping the same center.
- For 0.5<curvature<2, the radius needs to be extended beyond the
  inset/outset,
  as the thickness at the meeting point with the origin rect can be up
  to sqrt(2)
  thicker than the actual border width.
- Instead of rendering concave corners, we flip the vertex's center and
  outer points, to simplify computations.

Since corners with offset don't start/end at a point on the target
rect, and can overlap in some extreme situations, path intersection
logic is
used to create the result path. We intersect the target rect (without
any corners), together with two paths, each taking into account the
opposite diagonal corners, since diagonal corners cannot intersect.

Added a few helpers:
- Some helpers for RectVertex in path.cc
- Normalize functions for Vector2d.

Note that this is still insufficient in order to correctly draw concave
borders with a color, since the per-side color clipping needs to be
fixed (in a subsequent CL as this one is big enough).

Bevel and convex borders are rendered correctly now. The tests for
bevel+border can now use the same tests as the other variants use.

Bug: 399449172
Change-Id: Ib799423322c29270c678594eb6f6ed8a4a3a9a97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6368660
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434931}