Create ContouredRect, a superset of FloatRoundedRect

ContouredRect supports curvature and is built to correctly render
`corner-shape` (superellipse corners).

It is used instead of FloatRoundedRect whenever a rect can have custom
curvature.

Other places, e.g. canvas or some internal paint code, still uses
FloatRoundedRect directly.

This CL doesn't change existing behavior, and only extract the
curvature-specific bits out of FloatRoundedRect.

Many places are tagged with TODO, when a ContouredRect is simplified
into a FloatRoundedRect, which is a sign that curvature is not supported
for that code path.

Bug: 393145930
Change-Id: I54678bf132233b9bd52a9bf4b259d920a28f2300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6343681
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431456}