Emoji width correction doesn't run inside Web Workers

On macOS, Chrome's and Firefox's canvas measureText() reports Apple Color Emoji wider than the page paints it at small font sizes (see PLATFORM_BUGS.md). Pretext works around this once per font in getEmojiCorrection() (src/measurement.ts): it measures on canvas, and if that looks too wide, it measures a hidden span in document.body and subtracts the difference for every emoji.

A Web Worker has no document, so the correction silently stays 0 there. Text with emoji prepared in a worker would measure wider than what the page paints, so a worker computing heights could count more lines than the main thread draws. Without the correction our accuracy sweeps drop from 7680/7680 to 7660 in Chrome and 7652 in Firefox. I haven't checked that OffscreenCanvas inside a worker reports the same emoji widths as the page's canvas, but I'd expect it to.

This came up while thinking about laying out a very long chat on workers. One way out is to let the page read its correction for a font and hand it to the worker. Related: #34, and the ENGINE_FOLLOWUPS.md note that no canvas follows a worker's lang.