Add the support of offscreen canvas - #8554

• Closes https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE
• I have followed the instructions in the PR template

Hello,

This is a pull request to add the support of OffscreenCanvas

The context and my goal is to be able to run rerun (or any other library/sdk with egui GUI) into JupyterLite.

This adds a way to run an eframe app on an OffscreenCanvas owned by a Web Worker, instead of the <canvas> element in the page. The classic main-thread path is untouched: nothing changes unless you use the new entry point.

WebCanvas (crates/eframe/src/web/canvas.rs) is a small Html / Offscreen enum that the runner, the event wiring and both painters (glow and wgpu) now go through. The offscreen kind is what the new OffscreenCanvas / OffscreenCanvasRenderingContext2d web-sys features are for.

A worker has no DOM, and DOM events can't be constructed or structured-cloned into it, so the host page keeps the listeners and forwards raw pointer/wheel/modifier data as plain messages (web/host.rs on the Rust side). The worker parses those back into egui events (web_events.rs), and from there it is the same input pipeline as always.

Glyph/font loading and web_location() used to need window/document, they now fall back gracefully inside a worker, which is what lets the demo render there with its icons intact. WebInfo::offscreen_canvas tells the app which mode it is running in, and the demo's Backend panel shows it.

There is a demo page dedicated for offscreencanvas: web_demo/offscreen.html. To try it:

./scripts/build_demo_web.sh --glow
cd web_demo && python3 -m http.server 8765
# then open http://127.0.0.1:8765/offscreen.html