Make draw_box_shadow take a CSS-style box/offset/spread; add NonUniformRoundedRect

[nicoburns]

Summary

Breaking change to PaintScene::draw_box_shadow. AnyRender now takes the box casting the shadow plus the CSS parameters, and computes the shadow geometry itself. This works with the currently released vello 0.11 / vello_cpu 0.3 / vello_gpu 0.3. Switching the Vello backends to linebender/vello#1718's *_blurred_rounded_rect_in + invert is left to a follow-up (#113), which also removes the inset-shadow layers.

// before
fn draw_box_shadow(&mut self, transform, rect, brush, radius, std_dev);
// after
fn draw_box_shadow(
    &mut self,
    transform: Affine,
    box_shape: &NonUniformRoundedRect, // border box (outset) / padding box (inset)
    offset: Vec2,
    spread: f64,
    std_dev: f64,                      // CSS blur radius b => std_dev = b / 2
    brush: Color,
    kind: BoxShadowKind,               // Outset { clip_to_box: bool } | Inset
);

New anyrender::NonUniformRoundedRect (rect + NonUniformRoundedRectRadii { top_left, top_right, bottom_right, bottom_left: Vec2 }) is a kurbo::Shape for CSS-style boxes with elliptical per-corner radii.

• Fast paths: as_rect() returns Some when every corner is sharp, and as_rounded_rect() when every corner is circular and within kurbo's clamping, so backends can use their rect / rounded-rect fast paths.
• Paths: path_elements is a non-allocating iterator. perimeter is computed analytically.
• spread(s) follows the CSS spread rules:
  • Sharp corners stay sharp.
  • When shrinking, r' = max(r - s, 0).
  • When growing, r' = r + s, except that a radius r < s grows by s * (1 + (r/s - 1)^3) instead.
  • The result then goes through scale_overlapping_radii().
• Conversions: From<Rect> and From<RoundedRect>.

BoxShadowGeometry::new(box_shape, offset, spread, std_dev, kind) is shared by all backends. It computes:

• shadow: the box spread by ±spread and translated by offset. For inset shadows this is the unshadowed hole.
• area: the non-zero region to paint.
  • Outset: the shadow's 3σ extent, with the reversed box cut out when clip_to_box.
  • Inset: the box.

Backends:

	vello_cpu / vello_hybrid / WebGL	vello (classic)	Skia
std_dev == 0	push_clip_path(area) + fill_path(unblurred_path()). It's a single fill, so the non-isolated clip is safe	draw_unblurred (clip layer + fill)	native
blurred outset	push_clip_path(area) (if clipped) + fill_blurred_rounded_rect	draw_blurred_rounded_rect_in(area, ..)	native
blurred inset	draw_inset_with_layers: clip layer filled with brush, then a DestOut layer containing the blurred hole	same	native

• Vello radii: the Vello backends average the radii (average_radius()), because the blurred rounded rect only supports one radius.
• Skia: draws exact per-corner and elliptical radii. It uses clip_rrect(box, Difference / Intersect) and then a blurred RRect / DRRect, with no layer.

Recording / serialization:

• Shapes: stored as enum RecordedShape { Rect(Rect), RoundedRect(RoundedRect), Path(BezPath) }, chosen via as_rect() / as_rounded_rect(). Replayed scenes keep the fast paths.
• BoxShadowCommand: stores the new parameters.
• Archive version: ResourceManifest::CURRENT_VERSION is now 2, so version 1 archives won't deserialize.

Testing

• cargo clippy --workspace --all-targets -D warnings, cargo test --workspace (plus --all-features for anyrender / anyrender_serialize, and vello_cpu with and without multithreading), and a wasm32 webgl clippy of anyrender_vello_hybrid.
• New unit tests:
  • Spread radii: growing, shrinking, collapsing, and overlap scaling.
  • BoxShadowGeometry areas.
  • vello_cpu render tests for unblurred and blurred, outset and inset shadows.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/45dbc6c083354777aaa43442ce8ef8ab
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/45dbc6c083354777aaa43442ce8ef8ab?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

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