Support "blurred rounded rect"s with non-uniform corner radii

Nobody has claimed this yet.

Vello [currently supports](https://docs.rs/vello/latest/vello/struct.Scene.html#method.draw_blurred_rounded_rect) "blurred rounded rectangles" (aka drop shadows) with a single corner radius for the entire rectangle. However, CSS [specifies](https://drafts.csswg.org/css-backgrounds/#border-radius) the border-radius property with 8 distinct radii (separate x-axis and y-axis radii for each of the 4 corners). If we wish to fully support rendering CSS `box-shadow`s with Vello then we will need a version of `blurred_rounded_rect` that supports 8 radii.

The CSS spec is also adding the [`corner-shape`](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/corner-shape) property which further generalizes the corners to be super-ellipse rather than merely a regular ellipse.

An illustration of the problem with the current solution is below. Here the border (yellow/red/green/black) and outline (blue) are being rendered with the correct geometry, but the box shadow (grey) is being rendered with a single corner radius and thus doesn't match the shape of other parts of the node.

Start with Vello's Scene::draw_blurred_rounded_rect entry point and compare its current single-radius behavior with the CSS border-radius specification linked in the issue. Determine the API and geometry needed for eight independent radii, and clarify whether corner-shape support is in scope. Done means blurred rounded rectangles match the non-uniform corner geometry described by the issue.

Written by the indexing model from the issue text.