Feature: preloader function for fonts and SDF generation

[lojjic]

Font download and SDF generation is currently lazy, waiting until a TextMesh is first created and sync()-ed. This can cause a noticeable period of blankness when a scene is first rendered.

We should add a function that users can call to preload a font and optionally generate SDF for a set of initial glyphs. It would take a callback that is invoked when the font file is loaded, parsed, and the SDFs are ready. It would then be ready for TextMeshes to use without any noticeable delay.

Proposal:

import {preloadFont} from 'troika-3d-text'
showLoadingScreen()
preloadFont('path/to/font.woff', 'abcdefghijklmnopqrstuvwxyz', () => {
  showScene()
})