Avoid cloning owned font data when constructing font faces

[undermoonn]

• Closes N/A
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Summary

Reuse the Arc<FontData> already stored in FontDefinitions as the owning Blob for each font face.

Previously, blob_from_font_data cloned FontData. When the font uses Cow::Owned, this also cloned the entire underlying font byte buffer.

FontData already implements AsRef<[u8]>, so its existing Arc can be used directly as the stable owner required by FontCell and Skrifa.

Reproduction

A black-box eframe reproduction is available here:

https://github.com/undermoonn/egui-font-data-clone-repro

The repository contains two branches:

• main: crates.io eframe/epaint 0.35.0
• patched: the same application with only epaint and emath patched to this change

The application code and font are identical between the branches. It loads a
7.95 MiB Noto Sans SC font from disk using FontData::from_owned and renders
through glow/OpenGL.

Three Windows runs were measured for each branch:

Branch	Private bytes mean	Working set mean
main	99.46 MiB	85.61 MiB
patched	91.63 MiB	77.61 MiB
Reduction	7.83 MiB	8.00 MiB

The process-level reduction closely matches the 7.95 MiB font file.

Safety

FontCell retains its own strong Arc reference, so the font bytes remain alive
at a stable address for as long as Skrifa references them.

The shared data cannot be mutated in place. Any use of Arc::make_mut creates a
separate copy instead of modifying bytes referenced by an existing font face.

Testing

• cargo test --locked -p epaint --lib — 45 passed
• cargo clippy --locked -p epaint --all-targets --all-features -- -D warnings
• cargo check --locked -p epaint --no-default-features
• cargo fmt -p epaint -- --check
• Black-box eframe comparison linked above

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8320-epaint-share-font-data

View snapshot changes at kitdiff