clip-path: implement closest-corner and farthest-corner shape radii

[nicoburns]

Summary

Fixes CRASHes (todo!()) in css/css-masking/clip-path/clip-path-circle-closest-corner.html and clip-path-circle-farthest-corner.html (2 → 0 crashes; both now PASS).

Implements GenericShapeRadius::ClosestCorner/FarthestCorner in resolve_shape_radius per css-shapes-1: the radius is the distance from the shape's centre to the closest/farthest corner of the reference box, i.e. hypot of the per-axis closest/farthest side distances (min/max of offset and size - offset on each axis, clamped to ≥ 0 for closest-corner when the centre lies outside the box).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/a8bd4846bf354f33a6037fffaca1f583
Requested by: @nicoburns

WPT results

2 newly passing, 0 newly failing (net +2).

Full diff (2 changed tests)

+ Crash => Pass css/css-masking/clip-path/clip-path-circle-closest-corner.html
+ Crash => Pass css/css-masking/clip-path/clip-path-circle-farthest-corner.html

_{Generated by the WPT workflow.}

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