Use vello#1718's inverse blurred rounded rects for box shadows (follow-up to #117)

[nicoburns]

Summary

Follow-up to #117, and based on it. It upgrades to linebender/vello#1718, then switches the Vello backends to its *_blurred_rounded_rect_in methods with invert. As a result, blurred inset shadows no longer need compositing layers. The draw_box_shadow API from #117 is unchanged.

// #117 (released vello)
inset  => geometry.draw_inset_with_layers(..)            // clip layer + DestOut layer
outset => push_clip_path(area); fill_blurred_rounded_rect(rect, r, σ, false); pop_clip()
// this PR
fill_blurred_rounded_rect_in(&geometry.area, &geometry.shadow.rect, r, σ, geometry.is_inset())
// classic vello: draw_blurred_rounded_rect_in(&geometry.area, .., geometry.is_inset())

Unblurred shadows and Skia are unchanged from #117.

Do not merge yet: Cargo.toml has a [patch.crates-io] that points vello* and glifo at the linebender/vello#1718 branch. It needs removing once that's released.

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'.

• Disable automatic comment, CI, and merge conflict monitoring