experimental color emoji via opt-in egui_noto_emoji crate

[alltheseas]

This builds off of @lucasmerlin's color emoji draft PR.

I could not figure out how to commit to the open draft PR, so I am opening this one. Apologies.

I added noto open emoji set for the demo app. It works locally for me (see screenshots below) after downloading Noto fonts, and doing some format conversions.

Multiple Noto color emojis show in egui demo & render upon input.

Screenshots:

Summary

• Add built-in vibrant emoji pipeline
  • introduces GlyphColoring, atlas persistence, and the runtime
    loader that decodes a packed Noto emoji sheet, so 1 440 color
    emojis ship with egui and stay vibrant across themes/atlas
    resets.
• Add tooling to repack Noto emoji atlases
  • adds scripts/pack_noto_emoji.py plus .gitignore updates so we can
    regenerate the noto_{low,mid,high}.{atlas,bin} assets directly
    from the upstream zip.

Testing

1. (Optional) Regenerate the atlas if you tweak the sprites:

   ensure scripts/pack_noto_emoji.py exists (checkout the branch that
   added it)
   python3 scripts/pack_noto_emoji.py --zip noto-emoji-2.051.zip
   mv crates/epaint/assets/emoji/noto_low.png crates/epaint/assets/
   emoji/noto_low.atlas
   mv crates/epaint/assets/emoji/noto_mid.png crates/epaint/assets/
   emoji/noto_mid.atlas
   mv crates/epaint/assets/emoji/noto_high.png crates/epaint/assets/
   emoji/noto_high.atlas
   (The script always writes .png; renaming keeps the files on the non-
   LFS track the loader expects.)

2. Build & run:

   cargo check
   cargo run -p egui_demo_app --release --features "x11,wayland"
   In the demo window open one of the text input boxes ( I used EasyMark Editor in top menu) and enter emojis like 😀
   😍 🦀 to verify color rendering.

3. Snapshot tests (requires Git LFS assets):

   cargo test -p egui_demo_app --test test_demo_app
   Expect differences; once reviewed, re-run with UPDATE_SNAPSHOTS=1
   to refresh.

• Closes or makes progress on Color Emoji support #2551
• I have followed the instructions in the PR template

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/7707-alltheseas/color-emoji-tweaks

View snapshot changes at kitdiff

[lucasmerlin]

Hi! Awesome that you got it to work! Seems like numbers are always using emojis now though:

We should really keep the egui surface minimal though, egui should just have some api to register a image with a glyph, or maybe some kind of loader api to register them on demand. Then you could create a 3rd party crate for e.g. noto emoji that people could add to their project to get emoji support. Especially since egui will eventually get built in emoji support when we switch the font rendering implementation (#7694)

[alltheseas]

Building on @lucasmerlin feedback, emojis are rendered via optional crate approach.

• Stop emoji font from replacing digits – prevent the emoji atlas from
  overriding ASCII digits/#/* so numerals keep the text font while still
  loading other sprites.
• Add color glyph registration API – expose
  Context::register_color_glyph[_arc] plus plumbing so external crates can
  inject bitmap glyphs across all fonts.
• Move bundled emojis into optional crate – extracted the heavy emoji
  assets/loader into the new egui_noto_emoji crate and made the demo opt-
  in so core egui stays lean.
• Document optional emoji bundle – README/lib docs/changelogs updated with
  instructions to install egui_noto_emoji for color emojis.
• Allow overriding cached monochrome emoji glyphs – registering a color
  glyph now replaces any previously cached monochrome fallback, keeping
  emojis vibrant in all themes.
• Ignore multi-codepoint plan doc – .gitignore updated so the WIP design
  plan isn’t tracked/pushed.

[alltheseas]

Rebased onto latest main 2b792c1.

Fixed light/dark mode toggle breaking color emoji rendering 6778db8.

@lucasmerlin 👀