vello_cpu: use isolated clip layers for non-rect clips

[nicoburns]

Summary

#114 made vello_cpu's push_clip_layer use the non-isolated push_clip_path for non-rect clips when single-threaded. That double-counts anti-aliasing where several draws overlap the clip edge: each draw is clipped and blended on its own, so an edge pixel with coverage (a) ends up at (1-(1-a)^n). With an isolated layer, the content is drawn at full strength and the edge is anti-aliased once, when the layer is composited.

It shows up as a WPT regression in Blitz: css/css-masking/clip-path/clip-path-fillBox-1a.html (background, border and SVG content all overlap the clip-path: circle() edge; edge pixel 84 instead of 145, 364 px outside the fuzzy tolerance).

// push_clip_layer, vello_cpu
if let Some(rect) = clip.as_rect() { push_clip_rect(&rect) }   // unchanged; pop_clip
else                               { push_clip_layer(&path) }  // was: push_clip_path when !is_multi_threaded()

• Rect clips keep push_clip_rect. That is what newly passes 4 Blitz WPT tests (abspos-overflow-010, the two color-scheme-iframe-background-mismatch-* tests, fractional-scale-gradient-bg-obscure-red-bg), which were 1/255 off when rect clips went through a layer.
• Same behaviour with and without threads. vello_cpu now renders the same whether multi-threaded or not, so the is_multi_threaded() branch is gone.
• New test: overlapping_fills_in_path_clip_are_antialiased_once checks that 3 overlapping fills under a circle clip render the same as 1. It fails with push_clip_path.

vello_hybrid / WebGL still use push_clip_path for non-rect clips and have the same edge behaviour. Whether to isolate those is a separate question, because it needs a GPU benchmark.

Testing

• cargo clippy --workspace --all-targets -D warnings.
• cargo test -p anyrender_vello_cpu, with and without multithreading.
• Ran the 5 WPT tests above locally through Blitz (Draw box shadows via AnyRender's box-based draw_box_shadow; pass CSS boxes as NonUniformRoundedRect blitz#1024 with this change merged into anyrender#113). All 5 pass.

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

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