Draw box shadows via AnyRender's box-based draw_box_shadow; pass CSS boxes as NonUniformRoundedRect

[nicoburns]

Summary

Uses the new draw_box_shadow(transform, box_shape, offset, spread, std_dev, brush, kind) and NonUniformRoundedRect from DioxusLabs/anyrender#117, which works with the released vello 0.11 / vello_cpu 0.3. Blitz no longer computes any shadow geometry. Inset shadows lose their layers once DioxusLabs/anyrender#113 (vello#1718) lands; Blitz needs no further changes for that.

// outset (reversed, so the first shadow is on top)
scene.draw_box_shadow(self.transform, &border_box_shape, offset * scale, spread * scale,
                      blur * scale / 2.0, color, BoxShadowKind::Outset { clip_to_box });
// inset
scene.draw_box_shadow(self.transform, &padding_box_shape, ..., BoxShadowKind::Inset);

• Removed: max_shadow_rect, CssBox::shadow_clip*, NonUniformRoundedRectRadii::average, the blur == 0 && spread == 0 special case and its clip layer, and the inset DestOut layers. AnyRender now computes the CSS spread radii and handles unblurred shadows exactly.
• clip_to_box: this is opacity < 1 || background not opaque, the same condition as the old clip layer.
• Rendering changes, all CSS fixes:
  • The blur is now passed as σ = blur radius / 2. It was passed as σ = blur radius, so shadows were about twice as blurry as they should be.
  • The outset blur and the inset offset are now scaled by self.scale.
  • Inset shadows are painted in reverse order, so the first is on top.
  • A transparent inset shadow no longer stops the remaining shadows from being drawn: it used to return instead of continue.
  • Unblurred shadows with spread keep exact per-corner radii.
• CSS boxes as shapes: CssBox::{border,padding,content}_box_shape() return an anyrender::NonUniformRoundedRect. Backgrounds, background clips, overflow clips, masks and shadows use these instead of *_box_path(), so sharp-cornered boxes reach fill_rect / push_clip_rect and Skia's draw_rect / clip_rrect. Compound shapes (outlines, border rings) stay BezPath.

Do not merge yet: [patch.crates-io] points anyrender at the anyrender#117 branch. It can be removed once that's released.

Testing

cargo clippy --workspace and cargo test -p blitz-paint pass. I rendered a test page of inset and outset shadows, covering blur, spread, offset, radius and translucent backgrounds, at 1x and 2x on vello_cpu, vello classic (llvmpipe) and Skia raster:

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

WPT results

Subtests: 8 newly passing, 0 newly failing (net +8).

Full diff (8 changed tests)

+ FAIL => PASS  [1/1]  +1  css/CSS2/positioning/abspos-overflow-010.xht
+ FAIL => PASS  [1/1]  +1  css/css-backgrounds/box-shadow-radius-001.html
+ FAIL => PASS  [1/1]  +1  css/css-backgrounds/box-shadow-radius-002.html
+ FAIL => PASS  [1/1]  +1  css/css-backgrounds/css-box-shadow-001.html
+ FAIL => PASS  [1/1]  +1  css/css-borders/corner-shape/corner-shape-inset-shadow.html
+ FAIL => PASS  [1/1]  +1  css/css-color-adjust/rendering/dark-color-scheme/color-scheme-iframe-background-mismatch-alpha.html
+ FAIL => PASS  [1/1]  +1  css/css-color-adjust/rendering/dark-color-scheme/color-scheme-iframe-background-mismatch-opaque.html
+ FAIL => PASS  [1/1]  +1  css/css-transforms/fractional-scale-gradient-bg-obscure-red-bg.html

_{Generated by the WPT workflow.}

[staging-devin-ai-integration]

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

• Disable automatic comment, CI, and merge conflict monitoring