Rendering of corner-shape: bevel

This sets the infrastructure for rendering corner-shape,
by storing 4-float "CornerCurvature" on a FloatRoundedRect.

Currently only the special-case for "bevel" is handled, with the idea
of iterating on the different aspects of rendering as we go
along.

To properly render bevel (and any other curvature that's lower than
round), drawing a straight line from the outer/inner sides
doesn't create the right effect, because the diagonal distance between
those lines doesn't match the border width.

To correct that, while maintaining rendering within the initial
corners, the outer corner is shrunk to the point where the resulting
diagonal quad has a width equal to the border-width (or interpolating
between two border-widths).

See open spec issue: w3c/csswg-drafts#11610

Removed existing hand-coded SVG tests, and generating the refs using
a canvas polyfill instead.

This polyfill can be amended later on as we polish the spec.

Bug: 394059604
Change-Id: I9fb76dd7dc557ed10c40adecbb9f68f672de1acc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6250197
Reviewed-by: Philip Rogers <[email protected]>
Commit-Queue: Noam Rosenthal <[email protected]>
Reviewed-by: Fredrik Söderquist <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1421457}

Author: noamr