Support SVG filter effects in anyrender_svg

[nicoburns]

Summary

Addresses the "Filter effects" item of #88. Previously render_group silently dropped group.filters(); now filtered groups are rendered inside a layer with an anyrender::Filter attached, leaving actual filter evaluation to the backend (no rasterization, pure PaintScene abstraction).

New module anyrender_svg::filter converts a usvg filter chain into a single anyrender::Filter DAG:

pub(crate) fn to_anyrender_filter(filters: &[Arc<usvg::filter::Filter>]) -> Option<Filter>

• Maps all usvg primitives with a direct FilterEffect counterpart: blend, color matrix (incl. saturate/hueRotate/luminanceToAlpha), component transfer, composite (incl. arithmetic), convolve matrix, lighting, displacement map, drop shadow, flood, gaussian blur, morphology, offset, tile, turbulence.
• feMerge is lowered to a chain of Composite(Over) nodes.
• in="SourceAlpha" in a chained filter is synthesized via a helper ColorMatrix node; named result references resolve through a result map.
• Returns None (→ whole conversion fails) for the inexpressible cases: feImage, anisotropic blur/morphology/turbulence, non-square convolution kernels, non-white lighting colors.

In render_group, when a group has filters:

if !g.filters().is_empty() {
    match (filter::to_anyrender_filter(g.filters()), g.filters_bounding_box()) {
        (Some(f), Some(region)) => scene.push_layer(Mix::Normal, 1.0, transform, &region, Some(Arc::new(f)), None),
        _ => error_handler(scene, node), // report instead of silently dropping; render unfiltered
    }
}

The filter layer is pushed inside the existing clip/opacity/blend layer and clipped to the SVG filter region, so filtering applies to group content before the group's clip/opacity/blend composite — matching the SVG rendering model.

One small API extension in the anyrender crate: Filter::set_output is now pub, so multi-node graphs built outside the crate can select their output node.

Known approximations (documented in filter.rs): primitive subregions, color-interpolation-filters, convolve targetX/targetY/edgeMode/kernelUnitLength, and turbulence stitchTiles are ignored.

Unit tests cover blur, drop shadow, named-result references, merge lowering, and unsupported-case fallbacks.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/742382aca6144666b1a508e6897f72fc
Requested by: @nicoburns

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[staging-devin-ai-integration]

Tested this PR by rendering SVGs headlessly to PNG with anyrender_svg + anyrender_vello_cpu (--features filters), comparing filtered output against identical unfiltered controls (pre-PR, filters were silently ignored so filtered output was pixel-identical to the control).

feGaussianBlur (stdDeviation=3) — renders correctly

feDropShadow (dx=6 dy=6 stdDeviation=2) — renders correctly

Details & additional checks

• Blur: edge scanline shows a gradual alpha ramp (0→27→136→235→255) vs a hard 0→255 step in the control.
• Drop shadow: shadow-only region below-right of the circle contains translucent black pixels (e.g. rgba(0,0,0,118)) that are fully transparent in the control; diff bbox extends past the circle by the shadow offset.
• Unsupported filter fallback: an feImage filter invokes the custom error handler and the element still renders unfiltered (not blank), as intended.
• Note: vello_cpu applies layer filters only with the filters feature enabled and multithreading disabled.