vello_cpu: Cache the resolved image in Fine

[nicoburns]

Summary

Fixes the negative multithreaded scaling of cached glyph rendering in vello_cpu.

Before this change, Fine did the following for every EncodedPaint::Image command:

1. called resources.image_resolver.resolve(id)
2. cloned the Arc<Pixmap>
3. dropped the clone at the end of the command

All cached glyphs sample the same glyph-atlas page, so every worker thread hammered one shared refcount. In a perf profile at 4 threads:

• ImageRegistry::resolve was 16.6% of samples, 98% of it on the atomic increment.
• paint_fill was 24%, 90% of it just after the atomic decrement.
• The painters also suffered, because the pixmap's width, height and data pointer sit on the refcount's cache line.

Now Fine keeps the most recently resolved image:

resolved_image: Option<(ImageId, Arc<Pixmap>)>,
// per command:
if !matches!(&self.resolved_image, Some((cached, _)) if cached == id) {
    self.resolved_image = Some((*id, resolver.resolve(*id)?));
}
let pixmap: &Pixmap = &self.resolved_image.as_ref().unwrap().1;

This means one resolve per thread per image rather than one per command. ImageSource::Pixmap is borrowed instead of cloned.

Part 2 of 3 of the CPU glyph-cache speedups. It textually conflicts with the alpha-mask fast-path PR in fine/mod.rs; whichever merges second needs a trivial rebase.

Measurements

Throwaway probe: 1600×1200 target, ~9.3k glyphs, 14px, atlas_cache(true), mean of 60 warm frames. Render time, main → this branch:

threads	main	this branch
1	4.50 ms	3.27 ms
4	6.32 ms	1.89 ms
8	6.60 ms	1.22 ms

With the cache on, total frame time at 8 threads goes from 11.2 ms to 4.1 ms. Uncached rendering is unaffected.

cargo test -p vello_cpu -p vello_common passes.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/55344a6e161d48b4a3a8688fabe11d4c
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/55344a6e161d48b4a3a8688fabe11d4c?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